If, for example, you have 5 Democrats who can’t go along with this, that means you now need 9 Republicans willing to put their own standing in the party at risk.

I think this is the main reason. Also, you don’t become speaker just to be speaker. You do it to advance further, either to higher elected positions or, more likely, to lucrative party fundraising, consulting, speaking engagements, writing books, and/or private sector positions. In today’s world no speaker who is elected with the help of Democrats has a future in the Republican party, anywhere. It’s career suicide. Not just losing elected office. They will be blacklisted everywhere. It won’t happen.