Because that by far is the most likely outcome of a brokered convention, given the past history of the GOPe.
Too, even if the GOP-E "appear" to be supporting Cruz at the moment, to beat Trump, why is that bad? Do you really think Cruz will owe them Luciferian fealty forever? For a few million bucks in political funny-money?
None of us can answer for what the defeated remnants of the Democratic Party of 40 years ago will do, dispossessed by the neo-Stalinist Progs of their own party. They appear to be gravitating toward Donald Trump, who is really one of them as shown by his own vita; and their help would be appreciated by all of us if we can beat Beastwoman. But do we really want to walk away from our own principles and give up our own objectives, just to house people who lost their own party? It's they who should have to "settle" -- not we.
Yes, I know you're concerned that the PRC will a) Beat Donald, b) Beat Ted, c) Impose unvetted nominee (Yeb, etc.) on party faithful, someone who will be spat out by the base and defeated by Hee-a-Ree the Beastwoman. But those people will do whatever they do, and if they do work successfully to impose their heart's desire, they will destroy "their" party and make way for another .... perhaps after a long night of dictatorial misrule by the 'Rat Party, operating as a national-level RICO, an openly and frankly corrupt machine hated by all. And we can only do what honest people know to do, which is to vote for whoever we can, to try to overcome the scheming of the "party people" of both parties. That's all we can do, in the end.