There's no real market to San Juan PR
from the UK.
No UK - Puerto Rico business contacts.
No established holiday destinations.
No VFR market either.
Not an independent traveller destination.
No connection points that could not be reached just as easily through Miami or the Eastern Caribbean.
There are some cruise ships that leave from there, and there are periodic charters from the UK to San Juan that serve them. Other than that not much at all. Which is presumably what BA found when they tried it a few years ago. They can put you through Miami or Madrid all on OneWorld multiple times a day anyway.