Why can't ramjet diffusers be stacked to achieve a good overall pressure ratio?
A diffuser is simply an expanding piece of pipe that converts kinetic energy (i.e. velocity) to pressure energy (i.e. pressure) according to Bernoulli's principle (which is just a formulation of the law of conservation of energy for fluid flow).

The air enters the diffuser with the velocity corresponding to the speed of the aircraft, and must still retain some speed through the combustion chamber. So only the speed above that can be converted to pressure. Few hundred km/s is already more than the speed in the combustion chamber, so there is some pressure gain, and the engine starts to work at all, but only above M2 the pressure gain becomes comparable to what a trubo-compressor can provide.

Also diffuser is simply an expanding piece of pipe and stacking two expanding pieces of pipe behind each other is still just an expanding piece of pipe. And two expanding pieces of pipe with a narrowing piece in between is no better than one of them, because the pressure only depends on the initial and final cross-section and not on the shape in between.