Because the incumbents are hemorrhaging red ink because of VoIP, and their offerings come too little too late.
So at any opportunity they'll try to kill VoIP, or at least the competitors at first, then they'll let their own VoIP services whither so people will flock back to the three-9's reliability of circuit switched.
The issue with E-911 is NOT a technical issue at all. For example, Vonage can get E-911 to work just fine in Rhode Island of all places. I dial 911 and I get the actual emergency answer point and they get my address info.
Vonage uses facilities based carriers like Paetec and Focal, those already have interconnection to E-911 services in most every community they server.