It really is all about the "sentiment". IMO, everyone is so hyper focused on everything Trump says, does, etc, etc...but popular opinion still hasn't grasped how completely anti-establishment the sentiment has become...Rubio was sort of "groomed" for this run several years back, but those few years as a sellout politico ruined his chances in this anti-establishment backdrop. The GOP and media pundits are apoplectic that running 24/7 news blitzes to destroy Trump aren't having any effect. Again, nothing will change the minds of those who are voting against the status quo.
I fail to see how a Bernie supporter (anti-establishment) would see anything positive in Rubio. His "positives" were that he could repeat his canned soundbites on command.
Cruz is a Tea Party anti-establishment guy who gave in to nothing. Compromise is a dirty word in his vocabulary. He wasn't a "sellout" as you claim Rubio to be, and yet he has no chance against Trump in the primaries. Simply put, a lot of Republicans are bedazzled by celebrity and bluster.
Again, that's not really what it's about (celebrity and bluster)...I truly believe that the "base", if you want to call it that, had enough of the spineless GOP (and especially the "Romney's" of the party) and will not support a well connected and party favorite...Cruz is doing reasonably well and that is reflected in the two man race. It's also important to remember that after 8 years of any administration, the politician who runs a campaign AGAINST his predecessor will have alot of support. Trump is running that campaign as O. did against Dubya.