I'm not surprised. Ahsoka is a character who is neither mentioned or discussed in Episode II or III, so not having her for a whole season would be... appropriate to give our characters time to move on? Still, it's not like they couldn't give her a story that takes place elsewhere without interacting with the big mains.
And as I said, I think they will, be it in Rebels or some other yet to be determined programming. She is a very popular character and Disney won't squander the opportunity to expand the SW franchise to be more accessible for young girls.