Right. They don't have the same level of commitment from the US government.
SIPC is like Fannie Mae, where the US implies a backing without making a promise. When push comes to shove the US Government gets to choose whether to do a bailout on a case-by-case basis.
FDIC is not like that at all. The US explicitly and unconditionally backs them.
Look at it in political terms. FDIC is guaranteeing everyone's savings, rich or poor. You just can't let that fail. SIPC is guaranteeing a bunch of investments. If the class in power takes a dim view of bailing out a bunch of "wealthy speculators", the ball can definitely be dropped.
No private insurance company can provide such guarantees and keep then if the entire sector needs to be bailed out at the same time.