> What is the reason the Raspberry Pi is not open at a lower level?

Broadcom. Much of the Raspberry pi lower level software has been closed since its inception as Broadcom will not publicly release specs.

Probably they are constrained by their choice of using Broadcom SoCs, which happened because the project was started by Broadcom employees.

I can't speak specifically to the raspberry pi, but generally SOCs like it uses do work like that. Most customers are concerned with the chip's high-level capabilities and actively don't want to touch the lower-level stuff, as it's irrelevant to what they're making.