I think it's 2 things:
1) A lot of technologists find real estate abhorrent. They know it should be fixed but they don't want to be the ones to do it. That's fair. My thought is that the fundamental problem of exchanging money for land is pretty cool, even though all of the connotations of "real estate" kind of suck. So I try to think about it from first principles... what's the ideal way to exchange money for land?
2) It's complicated to get the data due to MLS regulations. Companies like yours help with this, but I believe app developers still have to apply to each MLS to legally get data from a vendor like yourself. It's not like building an app on the Twitter or Tumblr API, you have to call the MLS office and talk to real people (who are usually experts in real estate, not tech) and often pay them real money. Also, I'm guessing your IDX feeds don't always include sold properties or historical data.