Plenty of reason to not use SNI. First off - not everything is a web browser. While browser support for SNI is pretty good these days, other clients are far behind. That random app that connects to your web API has to also support SNI, which means some old PHP library somewhere has to as well. Good luck.

That and a surprising number of "regular" clients still seem to have issues with SNI. My numbers are quite dated, but even as recent as 3 years ago it was something like dropping 10% of traffic for a high traffic site I did A/B testing with. I'm guessing the number was even higher, but the client requested we immediately stop the test once it became apparent it was a major reachability issue affecting revenue.