The biggest one is that realtors exist entirely because of regulation a the state level.
After that is the extremely entrenched business interest in the status quo - a lot of people are making money off the current system, and any real disruption is going to cost 95-99% of the real estate industry their jobs.
As noted elsewhere, the level of regulation is such that even offering better data to buyers/sellers has a lot of regulatory overhead, including physical offices with real people.
