1. It took him long enough and he waited until it was a big deal. I don't believe his initial claim that he didn't know who David Duke was - if that's true, he's too damned stupid to be president (and I don't think he's stupid.) I think he's a fake conservative who assumed that all Republicans are racist and that a David Duke endorsement would help.
2. Cruz has the support of several liberal Republicans. So does Trump (Christie - I know there are others, but I don't pay much attention to or put much stock in endorsements.) I'm glad those who endorsed Cruz were able to put aside policy differences (and in the case of the Bushes, personal animosity) and support an actual conservative, as opposed to one who recently decided he wanted to play one on TV.
3. I've said this several times, but if Trump get's the nomination, I will support him. I don't trust him to do the right thing, but I trust Hillary to do the wrong thing. If he doesn't get to 1237, nobody owes him anything and all bets are off. (Right now, I'd say it's extremely likely he will get to 1237.)
They are only doing it to try and deny Trump a majority of delegates. If Cruz accomplishes that, they will turn around and stab him in the back. It's what RINOs do.
So we might well end up handing the nomination right back to the GOPe after seeing two anti-establishment candidates dominate the primary season.