One of the key advantages of Sky was that they showed highlights of Raw/Smackdown on Sky One - a channel you have to pay for, but one that's available in loads of households (certainly more than have Sky Sports). So it was a great way to bring in casual viewers, particularly kids, and then get them hooked.
Can BT Sport do something like that? You'd think that casual exposure is something WWE will want to continue, so they can continue to create new fans.
It's a tough nut to crack I think. Can't imagine very many people watch Raw/Smackdown live in the UK, so for most it's something they record and watch the next day. But how many people even do that, when you can immediately watch clips on YouTube? Reckon Sky have probably seen it's not worth the money anymore, while for BT it's more valuable because they need more content to drive subscriptions.