The number one rule for a National Party in selecting a presidential nominee is to select a candidate that can bring their party victory in the national election. If the powers that be in the RNC are thwarted by the success of a populist candidate, who will not likely win the national election, (and in Trump’s case might, also, take the Party in unpredictable directions) they will do whatever they can to advance the chances of one of their candidates who is more predictable and more importantly, will have a better chance of winning the national election.
The RNC, in doing this, still is bound, however, by the extent to which its constituency will go along. The current constituency is made up of at least a few very important components. One component is a very strong anti-establishment bent. Thus we see the anti-establishment candidates doing relatively well (quite well in Trump’s case). But the RNC will always prefer backing an establishment candidate. Another very strong component of Repub constituency is the hard-right crowd, who prefer candidates that express hard-right stances across a broad range of issues. The RNC will often prefer a more moderate candidate, as the more moderate candidate would be more likely bring a better chance of victory in the general election. But they can’t get a moderate candidate nominated if the constituency is too hard-right. The other strong component is the religious one. It is very important to many Repubs that the candidate have a strong Christian orientation, of some sort.
So one of the reasons, I think that the RNC will prefer Rubio is that he represents the establishment, even if it may be less so than Jeb or Scott or John. (less, simply because he has been in the system not quite as long as those guys).
Also, Rubio is very religious, which will satisfy the eaten up with religion part of the constituency but will not represent as much of a liability in the general election as someone like Huckabee, who is all religion.
Also, in regards to, at least appearing moderate, Rubio has vastly polished his game, so that he will be able to present himself, to some, as moderate, despite taking hard right stances in actuality.
The RNC will prefer someone from a swing state, if they can get one. Rubio is from Florida. If they can win Fla. in the general, that will be significant. Kasich with Ohio, might be better, but he is too moderate for the constituency.
Also, to win the general, the Repub candidate needs to appeal to not only independent and casual voters but, additionally who can get some portion of populations that predominately vote Democratic.
As to the latter point, Rubio is physically attractive. That counts for something. Some women (both young and old) will vote for him because he is a sweet looking handsome man. (He could leach off even more of the women vote by selecting someone like Fiorina as a running mate. She would have the additional advantage of being a viable attack dog on Hillary who could not be accused of being sexist, because she is a woman.)
Also, being Hispanic, Rubio might leach off some of the Hispanic vote from the Democrats.
Anything, could happen, but if Walker and Bush continue to go nowhere, big money will eventually switch its allegiance to Rubio. And if Trump hasn’t extraordinarily upped his organizational and substance game, by then, (or if he is forced out of the Repub race, in some way), then I think that Rubio will get the nomination. He will need to win in Iowa, because he probably won’t win in New Hampshire, but I think Iowa is do-able for him.