> Why shouldn't mom-and-pop get the choice to invest in SEC regulated companies?

When retail, i.e. unaccredited investors, get hurt (a) there are more of them and (b) it more often than not puts them into dire, sometimes existential, straits. The latter leads to political backlash and instability, e.g. the 1930s.

Basically, you must have a nest egg to act as a safety cushion before you can invest in risky things. This makes sense on another level, in that doing diligence on un-registered companies is expensive. That, in turn, drives a minimum practical investment size.