Not true, firstly 'framework' never tried Coreboot they have provided boards to coreboot devs however, who've had issues including accidentally bricking the board. New boards have been shipped, and the origional fixed. They haven't given up though completely and the work is still being done. There was a recent talk that had to be canceled due to the speaker getting sick on this subject. Work is likely still being done, but is slow, and will continue to be so.
Still though, there's no evidence that framework has ever had or ever will have the intention of including core-boot by default. They're simply supporting coreboot devs to make it compatable
The reasoning for why the others have it is likely due to system76 and nova customs using mostly generic parts like the shell and motherboard along with being around longer. It's great to support a company that's ensuring coreboot works on their system however and pushing towards a non-proprietary future