Same as only having 4 layers per pad, it was a development decision. Ultimately this is a standalone machine with inherent limitations and part of the joy of working with these machines is finding ways to work with the limitations, which often leads to interesting and creative results. Many people also prefer having these limits as it means you don't get option overload.
4 FX on a pad? okay, I really need more, So maybe I can bounce some of the FX using flatten pad, maybe use some of the additional drum FX, perhaps I can use the lfo to simulate that tape wobble instead of using the flavor effect, etc etc.
If you find those limitations too limiting, then using a computer + DAW is maybe the way forward as all standalone hardware has fairly strict limits, and TBF the MPCs are now probably the least restrictive.