It never made sense not opening everything up from the start - did they really thing someone would just clone it and made bank if they themselves usually struggled to make the whole thing work financially.
In my opinion it was most likely the combination of the combination of three things:
1) The race to release the Jolla 1 ASAP back in 2013, resulting in a messy codebase and systems not setup for community to contribute.
2) Then clueless investors got involved, especially when they needed emergency funding after the Jolla Tablet debacle in IIRC ~2015, blocking Jolla from opening the full source.
3) Constant firefighting preventing engineers on actually opening things up and setting things up for people to contribute & actually review the contributions in timely manner.
So good to see things finally improving. :)