My preferred tool for house hunting is Redfin, but one area of frustration is that large geographic areas are apparently not served by Redfin (see screenshot below, gray areas are unserved). Why does Redfin not serve those areas? Or, if it does, how can I get access?
To answer your question, we should first examine the structure of the real estate industry.
In the United States, real estate is decentralized. This means there is no central body that warehouses property listings nationwide. Instead there are 600+ multiple listing services (MLS). Each has its own territory and each manages its own database of listings.
Brokerages like Redfin can publish these listings though agreements with individual MLS. Many brokerages serve only a handful of communities, so this system works for them. They set up one or two agreements with the local MLS and market the listings to local clients.
Redfin seeks clients nationwide. But to market listings, it still needs to establish individual agreements with each of the 600+ MLS. The gaps in coverage you see are areas where Redfin does not yet have an agreement with the local MLS.
But what about Zillow?
Some companies use other methods to access MLS listings. Zillow, for example, syndicates some of its listings. This means they not acquired directly from an MLS but from a third-party service that collects and distributes listings. This simplifies the process to gain access to listings and allows for a more complete coverage area.
Why doesn't Redfin syndicate?
Syndication can have some disadvantages. For example a Phoenix-area MLS, ARMLS, announced in July 2018 it would introduce a delayed listing status whereby agents could post listings to the MLS before they are sent out to syndication services. As a brokerage with direct access to the MLS, Redfin would have access to these listings as soon as they are posted. Zillow would have them on a delay.