For many companies releasing their code is not something they would usually consider, regardless of whether it offers a competitive advantage or not. It's just not in the culture. They don't see what they have to gain by releasing the code, but they worry that it may create issues if they do.

So basically if you want to convince these companies to open source some of their components "what do you have to lose?" is not good enough, you have to give them an actual incentive. I suspect that outside of places like HN very few people really care about Broadcom's binary blob in the rpi.