A normal database isn't able to be easily disseminated by the people who need access to this data, which is enormous. Then who gets to host it? Real Estate associations are too many, too fragmented, too entrenched to agree on one or even a handful of companies with the privilege to host this information.

A blockchain (assuming you can get buy-in/critical mass from the agencies) has the ability to align some financial incentives of all interested parties to open up access to real estate listings while still holding ownership and profiting from that access.

Instead of real estate companies protecting their listing data, they could offer access to their listing data in exchange for a fee or revenue share. Instead of out-of-date listings parading as up-to-date, there's a public temporal record of updates. Instead of buyers getting restricted or partial information, they can obtain access to the listing data via alternate sources.