As far as I can tell, it's because they had special access to MLS listing data. But with Zillow and Redfin, all that is pretty much moot.
Not moot at all as zillow, realtor.com, Redfin Etc all need to contract with mls in order to get access. Not every municipality gives them access
“But with Zillow and Redfin, all that is pretty much moot.”
I think (I could be wrong) that most of the data on Zillow, Redfin and Realtor.com orginate from MLS data, or the brokers who put the data on the MLS.