My research on these fish is that they just sort of die without any warning. Sounds like they earn their "difficult" rating.
The LFS where my brother lives recently had one - it had been in a doctor's office tank for 3-4 years, ate frozen, didn't bother corals, and was on sale for $60. So he of course grabbed it, and it's done well for him. I think the key is getting one that knows how to be a "tank fish". But he got lucky - very few people are going to want to sell a happy, eating regal angel.
Sorry for the loss. Sounds like you did the best you could. Some fish just don't make it. I doubt there was anything more you could have done.