Because they will not take Cruz's name off the list of candidates. Even if it's all true.
What they will do is hope that the issue being brought up is enough to discourage you from voting.
This is what Trump said would happen. He knows what tactics they use. The only one that can change the 'effectiveness' of that tactic is YOU.
Stick by your candidate, whomever that is. If that candidate becomes the nominee, vote for him(her). If not, then vote for the next best choice. Otherwise you are just helping the Dem candidate win.
If Cruz was the nominee the Democrats would keep him off the general election ballot in key states. He would have to sue, and it could drag on past the election. Gore v. Bush on steroids.