^Durant lost multiple viabilities with the rotation, such as Twins, Special Metal, Alph Lithograph and Rotomo UD, making it truly unplayable in the new format.
Additionally, consider that the program has a casual setting, much like a league, where all sets are allowed and you just play for fun. It also has a lot of people who likely just want to try out the game and get "basic color" theme decks, all of which are set in HG/SS era. Telling these people they can't play makes no sense when the idea is to grow the community. Additionally, this doesn't work as well as in the real life version for rotations because borrowing cards is impossible, trading is difficult at best, money cannot get you single cards and their isn't enoug hreward for becoming competitve (winning games vs, the prizes offered at tournaments).
So yeha, they'll have to add freely available BLW-on theme decks and make the competitve circut better before they can even think about rotating, and even then, the best solution may be to ladder lock formats (modified as Expert, unlimited as Novice).