When the person is associated with the candidate, it might be a good idea to distance oneself from such remarks.
Walker, of 2.59 grade point average fame, does not need to be associated with insulting the intelligence of a business genius like Donald Trump.
This is the second time Walker has spoken out against Trump, the first time denouncing him over McCain.
Cruz is competing against Trump as well. He does not engage in such attacks.
You do understand competing right? Cruz will ultimately need to distinguish himself from Trump and say why we should vote for Cruz rather than Trump or Walker or etc as each candidate will need to do.
So I would not call not apologizing to Trump for anything a mistake on Walker’s part. How Trump’s style wears the voting public remains to be seen.