Sky's approach to sport changed when Comcast took them over in 2018 and their approach deviated from trying to conquer the sports TV rights sector to focusing on entertainment.
They lost the rights to the Top14 and La Liga (Spanish football league) around this time. Both league's rights holders expected Sky to be willing to pay silly money for the rights, only to find they weren't willing to offer even close to what they had been paying. La Liga went to Eleven Sport, which soon collapsed and have ended up on Premier Sports. In the case of the Top14, where viewing figures rarely topped 15k, nobody has been willing to come close to what Canal+ wanted (they brought the international distribution rights as part of their domestic TV deal), so ultimately nobody has picked up the rights and it's unlikely to change until the current deal ends in 2023.
With the Pro14, the league wanted one broadcaster showing every game. Sky weren't willing to do that (viewing figures aren't much better, some games on Premier Sport attract viewing figures below 10k) so the rights went to Premier Sport.
If PRL have turned down a £40m offer from BT in hope that Sky will top it, they either know something I don't or are living in Cloud Cuckoo Land...