It puzzles me that many people don't yet know about Opus. Let me quote the FAQ :
"Does Opus make all those other lossy codecs obsolete?
From a technical point of view (loss, delay, bitrates, ...) Opus renders Speex obsolete and should also replace Vorbis and the common proprietary codecs too (e.g. AAC, MP3, ...)."
I.e. in practice - in my main archive I use FLAC. On some portable players and etc. I use Opus encoded from that FLAC.
That's why I always try to buy music in FLAC when possible and stores like Bandcamp are great for it.
I'd imagine they consider what they have is good enough considering the backwards compatability issue it'd likely introduce.
Just a guess but I bet it's because the cost would be higher than the extra revenue it would generate.
And twenty years from now it's going to be hard because you'll have to scrounge the gear from a museum instead of it being available for a reasonable price from eBay or borrowing it off somebody who kept it in the cupboard after upgrading to modern digital gear. So I'm glad Monty did it in that era where the gear was still available.