Publishers rejecting series - are they common?

I just got a rejection from a publisher. She loved the story, but didn't like that a cliffhanger I only added in the final chapter. Think something like the first book of Bobiverse, where an alien threat is presented but never resolved. It'd be hard to end the novel without it, though. "The Aliens just fled the system for no reason."

The story of my novel is self-contained, and I didn't even query it as a series. In the end, she told me they don't publish series.

So, are series only good for self-publishing these days?