Firstly, I understand the need to use hypergolics over cryogenics as part of Dragon 2's abort and landing sequence - they provide "instant on" thrust and are throttlable with very wide margins (SuperDraco itself can throttle down to 20%).
But why use Nitrogen Tetroxide & Monomethyl Hydrazine specifically? Newer chemicals such as AF-M315E (which will be tested as part of the Green Propellant Infusion Mission) offer some pretty cool advantages. Consider the numerous downsides Hydrazine possesses:
Highly toxic. Hazmat suits are a requirement when handling Hydrazine. It's toxic in very low concentrations and also carcinogenic. This slows loading & unloading times and increases cost and complexity significantly - something that is diametrically opposite to SpaceX's goals.
Costly. In 1990, NASA was paying $17/kg for Hydrazine. I highly doubt it has become cheaper since then. Stricter environmental & handling regulations will only have increased the cost, and when Dragon 2 holds a few hundred kilograms of it, that cost stacks up.
Low specific impulse. SuperDraco has an Isp of 235 seconds, which is terrible. AF-M315E has "50% increased performance". This could translate into a number of options for SpaceX. They could reduce the weight of Dragon 2 significantly, which could make recovery of Falcon 9 easier, alternatively, it could allow them to keep the same weight and pack more scientific instrumentation or cargo aboard. Another option is keeping the same amount of propellant and allowing for a longer, less aggressive landing burn or providing more maneuvering margin during abort situations.
Fiddly. Hydrazine has to be kept warm too - which requires heaters which requires power.
Now, I realize...
that AF-M315E is still relatively cutting edge, but so is Dragon 2. NASA is requiring that landings take place under parachutes for the time being, which leaves ample R&D time (something SpaceX are fond of) for Dragon integration with this newer chemical.
Can anyone explain this decision to prefer Hydrazine? Is a greener, more performant propellant in the upgrade line?