A problem with trusses inside a liquid is that you have the possibility to set up currents and cavitations in the fluid that can cause it to move in ways you don't want. Without the trusses, the fluid moves (moderately) smoothly in the direction of the nozzle. With the trusses, it can set up eddy currents and (in extreme cases) voids in the liquid.
When you have a pressurized vessel, you don't really want any extra pressures that you can't expect. The collapse of a void/cavitation can set up pressure waves that may cause over-pressure in a weak spot in the hull or, in this case, uneven burning of the fuel.
In an extreme case of many trusses, you might end up with an unintended baffle system, where it restricts the fuel flow so much that it slows the burn, potentially causing the engine to stall. Semi-tankers use baffles to help prevent sloshing so they can brake and accelerate easier. In the case of a rocket, you aren't stopping and starting a lot, so sloshing shouldn't be a problem, especially with essentially a single force acting on it in the direction the fluid needs to go.
Also, rockets have a problem named after them: The Tyranny of the Rocket Equation. It says that the more weight you have, the more fuel you need and the more fuel you need, the more fuel you need to lift the more fuel you need to lift the weight of the fuel you need to lift the other parts of your rocket. Simple, right? Trusses add weight to the rocket, so if you can do without them, you're better off. As other people mention, the skin of the tank and ship are more than enough to deal with the pressures and forces involved. If they weren't, the engineers would have added the necessary trusses. ;-) Or used a stronger material.
When you have a reusable tank that is recovered after a mission, you have to inspect it inside and out. With more surfaces and joints/seams/welds to inspect, you increase the time, effort, and cost to do the inspection. If there are any coatings to prevent the fluids from eating away the tank, you increase the cost of originally applying and any need for reapplying them by adding structure to the inside of the tank.