The Speaker can be anyone that gets the votes, doesn't even have to be a member of Congress. They can be from the minority party. In theory they could choose you or me.

You need 218(?) votes to win.

All 212 Democrats vote for the Democratic candidate every time, so McCarthy is not getting any of those.

So if just a handful of Republicans vote for another candidate, McCarthy gets less than 50% and doesn't win. So then they need to do a revote, because further House business can't proceed until they have a Speaker - it's the very first thing in the legislative procedure for the year.

Note that there hasn't been more than one round of voting in ~100 years, so nobody is really prepared to deal with it. And this is all protocol, not law, so the House could change it after this session. But they have to elect someone first...