So here's why

Facts
-Maison sisters are from Hoenn
-They moved to Kalos and opened Maison there
-It was a success so they opened one in Hoenn too
-They started building frontier in ORAS
-They plan to go back to Kalos
-Their battle theme in ORAS is frontier brain theme.

After you defeated Morgan in ORAS she says

"You really are the Trainer I thought you were, aren’t you now, pet? Heh heh... Might I ask something? Would you consider becoming the owner here? Me sisters and I have to be returning to our Battle Maison in Kalos someday. I suppose we’d feel safe trusting you with the Battle Maison name. There would also be the matter of our plans to build an even more impressive battle facility in the area. If we get our ducks in a row now, we may make oodles when we leave here. No more talk about it now, but do consider the idea, won’t you, pet? Well then, I wait eagerly for our next battle and for your favorable reply!"

As for the question "why there's no BF in Hoenn then?

Beacuse GF want you to buy next game. This way there'll be a nice connection between Kalos and Hoenn and also it'll be like in the originals BF comes in the third installment