There is no evidence that the Republican establishment thinks Rubio is more likely to beat Trump than Cruz. There is plenty of evidence that many in the establishment find Cruz no better than Trump. Some may even prefer Trump to Cruz. Trump at least claims to be in favor of "flexibility" in his positions. Cruz has an actual record of not being flexible.
That said, if you want evidence as to why they might think Rubio is in a better position than Cruz, note that going into Super Saturday, Rubio and Cruz were underperforming by about the same amount. Yes, Cruz had more delegates, but he got them in places where he should have won. For example, Cruz won a plurality of Texas voters. Texas alone gave him a hundred more delegates than Rubio. But Cruz was expected to win his home state of Texas. If Rubio wins his home state of Florida on March 15th, he will make up most of that gap.
Cruz: 236 of 384 delegates; 148 short
Rubio: 112 of 242 delegates; only 130 short
Alternate source: http://cookpolitical.com/story/9326
Of course, these calculations might look different after today's elections. Rubio may well fall further behind Cruz.
And of course, there is the point that Rubio would probably get more of the Kasich supporters than Cruz would.
Another issue is that it is not evident that coalescing around either candidate will be successful at this point. Some recent polling has Trump defeating either Rubio or Cruz in a two person race. Perhaps both should stay in the race so as to divert delegates from Trump and keep him under the magic number of 1237 delegates (the majority of the delegates to the convention).
Oh, and Rubio is generally considered a better general election candidate. He outperforms Cruz in both polls and in general perception.
Polling source: http://www.realclearpolitics.com/epolls/2016/president/2016_presidential_race.html