I still can't (personally*) see an advantage to ApplePay on the Tube, but for many people, myself included, it's simply more convenient. I don't have a CPC, and can't get one, but I can link a non-contactless card to my ApplePay. I also quite often have my phone in my hand, so it saves digging my wallet out.
(*It's still slower, even when you get it activated before you get to the gateline. It's still a pain, and you still need to have sufficient battery on your device. Caught a customer out yesterday evening!)
I have a CPC and Apple Pay, and have used both in London. I prefer Apple Pay, because:
1) It's easy to get my phone out and "arm" Apple Pay on the approach to the gateline. It's less easy to dig around in my wallet for the card.
2) I would often have my phone out in these circumstances, it's reasonably natural to do so. By contrast, I'd be highly unlikely to have my bank card out when going through barriers on the tube. I also would be highly unlikely to have the rest of my wallet out in this situation.
3) Apple Pay is secure. CPC is not.
4) I think it would be much easier for someone to steal my card than it would be to steal my phone. Tube stations can be busy, of course, and it must be relatively easy for an opportunist to pick up a CPC without the user noticing, whereas a phone would be much harder I reckon.
5) It feels pretty cool. I know it's a frivolous reason, but it does feel very futuristic.