A plurality of Republican voters aren’t conservative.
For many reasons.
So Rubio can get a lot of votes if he’s the last guy standing from that group of candidates.
And it’s not that they are not conservative so much as they are not knee deep in the details like most of us are. They lean to the right and may have strong feelings, but also are susceptible to media narratives and do not necessarily act on the red flags on certain issues. They’ll hear mostly what they like and make snap decisions on likability and trustworthiness. There’s nothing at surface level that is problematic about Rubio, unless you do not like his style of fast speech.