1) Rubio will be a much better GOP opponent against Hillary than the more combattive Ted Cruz. Cruz would have been great against Obama in 2012 where energizing and turning out the (angry) base is important. Against Hillary (female candidate), that might not work. Rubio OTOH, could duplicate what Obama did in 2008 vs Hillary and run on Hope and Change against the partisan and racially divisive Obama administration and Democratic Party. Do the voters want 4 more years of divisive, polarized politics under Obama-Clinton? Rubio has better credibility running as a non-divisive candidate over Ted Cruz.
In short: You go to war with Ted Cruz. You unite the country with Marco Rubio.
2) Only way Rubio has a shot at the 2016 nomination is if the Gang of 8 bill fails, and he proposes a better plan that is palatable to the base. Hopefully, something like this one.