Cruz is getting that support because his entire record in government is that of a conservative - not a conservative who is as popular with voters as Trump, but still a 97% to 100% conservative if you look at his record instead of at campaign rhetoric. However, election dynamics change when there are only two candidates left. I do not want Kasich to drop out and leave just Cruz, but neither do I want Cruz to drop out and leave just Kasich - unless both go at the same time.
Don't be so confident that Trump would crush Kasich, the last man standing, as easily as he crushed the rest of them. Trump had the advantage of divided opposition before. With that gone, the dynamic could be different. If Kasich support jumps in these final few primaries, that can be used as justification for nominating him if Trump falls short. I want to avoid that disaster, and the best way to avoid it is for both Cruz and Kasich to stay in.
[Note: I do not consider a Cruz nomination a disaster, but I consider it extremely unlikely. Cruz thinks he can play the establishment, but I expect them to betray him if Cruz stops Trump.]
The Establishment has seen that and now are backing two candidates, one representing the anti-establishment view (Cruz) the other one the establishment one (Kasich).
I want Kasich to drop out because he cannot win the majority of the delegates and is therefore only remaining in the race to fight a convention battle.
Likewise with Cruz when he is mathematically eliminated.
I doubt few Cruz voters would vote for Kasich, I know Trump voters wouldn't.
What the establishment is attempting to do is what it has always done to get their guys elected, divide conservatives, in this case religious conservatives (Cruz) and popularist ones (Trump), while Kasich slides through untouched.
If Kasich were leading the RNC would be putting all kinds of pressure for both Trump and Cruz to drop out for the good of the Party.