Why could the Lunar Ascent Engine be used only once?

The Lunar Ascent Engine was used during the Apollo missions to ascent from the lunar surface back to the Command and Service Module. There was one risky aspect of that engine: it was not possible to fire it repeatedly. Each ascent engine used on the Apollo missions was fired for the first time on the Moon.

There was of course testing done by the engine supplier (on units that never went to space), also the ascent engine was tested during early Apollo flights, but it was always a single purpose product. The actual flight hardware for each mission could not have been tested. Being a mission critical component, this looks insecure at least.

Why was the engine manufactured like that? The Wikipedia article is explaining it this way:

... the fuel and oxidizer were so corrosive that the engines had to be rebuilt after each firing. This meant that each ascent from the Moon was performed with an engine that had not been test-fired prior to flight.

However, identical fuel and oxidizer was used also on the Lunar Descent Stage and the CSM propulsion system. These 2 engines were fired repeatedly during the mission with the same propellants. So the actual problem must have been in the material, which was different from the other engines.

If it's the case, why was such material chosen, despite the fact that no testing of the flight unit could have been done? If the reason was mass reduction, then mass can be taken from less important components.

Contrary to what's commonly stated, I believe the ascent engines were fired for a limited period of time before installation.

According to a Boeing document titled "Apollo Spacecraft Engine Specific Impulse Part II dated October 1968 (bold mine):

the injector and valve assembly are first calibrated and then acceptance test fired in a water cooled steel chamber with an ablative liner. A single compatibility test of 460 second duration is then carried out. Finally, the injector and valve assembly are then assembled with their flight chamber and the complete engine is acceptance tested...

The description of the acceptance tests for the complete engine includes:

The engine and its thrust measuring rig are mounted horizontally in a capsule. Altitude pressure is obtained by a steam ejector and maintained during engine firing by an exhaust driven diffuser. Propellant tanks are pressurized with helium. There is provision for both temperature conditioning and helium saturating the propellants. A minimum of two satisfactory engine acceptance tests of 15 second duration have to be carried out. ... Throat and exit area measurements are taken prior to the first test and after the last in each test series.

It's not 100% clear to me that the nozzle isn't replaced after this test, but it seems like they wouldn't limit the burn time if they intended to replace them. In any case, the valves and injector are really the tricky bits, and those are well tested. Like the SPS and LM descent engine, the ascent engine was pressure-fed rather than pumped, with redundant propellant valves, so as long as the plumbing was good and one set of valves worked, the engine would fire.

If it's the case, why was such material chosen, despite the fact that no testing of the flight unit could have been done? If the reason was mass reduction, then mass can be taken from less important components.

The LM was over its mass specification all the way to Apollo 10, so it wasn't as simple as taking mass from other components.

The ascent engine itself was designed to be restartable. (In fact, a second burn was how the empty ascent stages were crashed into the moon.) It was the helium pressurization system that fed the fuel and oxidizer which had problems restarting.

The ascent engine, shown in figure 2, is a fixed-injector, restartable, bipropellant rocket engine that has an ablatively cooled combustion chamber, throat, and nozzle extension.

In the engine itself, there were problems with the injector, and ultimately it was re-designed and manufactured by a second vendor. In contrast, the helium pressurization system had many problems:

• Ten helium tanks were manufactured 0.1 inch too small. These were used as test articles.
• The original vendor of the helium solenoid latching valve could not meet pressure or leakage requirements. Under a new vendor, the O-rings showed chafing when the valves were repeatedly cycled. Testing of this valve at White Sands also showed problems with this valve remaining open when rapidly cycled. There were also problems brazing this valve into several of the LMs.
• The primary and backup vendors of the helium pressure regulator both had problems with lockup. The primary vendor's regulator locked up repeatedly. The backup vendor's regulator locked up once and had oscillations during testing, but were deemed within an acceptable range and used.
• Some of the squib valves had faulty brazing joints.

The ascent engine was tested at White Sands on the PA-1 test rig. Five "flight-qualified" engines and six thrust chambers were used among all the tests. I find no evidence that these test engines were re-used for actual flights, nor that hot-fire testing of production engines or their components was done. Test series #4 was 45 runs of "LM-1 mission duty cycle, restarts, engine stability, propellant depletion, off-nominal performance".

The unmanned Apollo 5 mission fired the ascent engine twice. The first firing was 60 seconds in duration. The second firing occurred 1-1/2 hours later, and continued to propellant depletion. The firing time was 40 seconds less than predicted. Later analysis concluded that at least 10 seconds of this missing time was attributable to problems in the helium system.

Apollo 9 was the first manned mission with the lunar module. It ran one manned 3 second firing of the ascent stage, followed by an unmanned firing of the ascent stage to depletion. During the first 290 seconds of the 362.3 seconds of the second firing, the system was significantly under-pressure. This was attributed to a malfunction of the helium pressure regulator.

Apollo 10 had an unexpected low fuel warning light, during the ascent from the near-touchdown. This was attributed to an ullage issue with the tanks that were deliberately filled half-full; not an issue with the helium system.

Remaining flights had no problems with the ascent propulsion system.

Update: The existance of time limits on pressurization of the LM propulsion systems is confirmed by the Apollo Operations Handbook: Lunar Module vol. 1, section 2.8.6:

• The ASCENT He PRESS switch should not be actuated longer than 24 hours before termination of ascent engine operation. The ascent pressurization valves are designed to operate for only 24 hours after exposure to propellant vapors. Ex­ceeding this limit may cause ascent valve failure.

• The DES START He PRESS and DES PRPLNT ISOL VLV switches should not be actuated longer than 3.5 days before termination of descent engine operation. The descent pressurization valves are designed to operate for only 3.5 days after exposure to propellant vapors. Exceeding this limit may cause descent valve failure.