[lpmay]

It's because the speed of sound (in any material) is much slower than the speed of light (even in high dk materials). To make a selective and low loss filter you want to use resonators which have dimensions which are a large fraction of a wavelength (i.e. a quarter wave). When the wave speed is slower, a wavelength is compressed into less space ( cycles per second / meters per second ) which is what enables the miniaturization.

I believe the acoustic loss in many of these materials is also lower than the dielectric loss in most materials, which also improves filter q and achievable selectivity.