First, I agree with this premise, I also think that this is the most important issue facing America today. I tell people that there will be no capitalist economy to deregulate or a limited Constitutional Republic to defend if our country becomes a political and cultural annex of Mexico and Central America.
So my objection isn't to the fact that Trump doesn't make repealing affirmative action laws a focus of his campaign. I am bothered by the fact that he says that he has no objection to affirmative action in principle. He could have said that he opposes affirmative action, then moved on to the next topic. Instead, he chose to say that he doesn't even oppose it. It's one thing to pick your fights and not make secondary issues a priority, it's another to be on the wrong side on those issues.
The people who oppose amnesty for illegals generally also oppose affirmative action, so which of his supporters would he have alienated by speaking out on the subject?
Notice he said he has "no objection to affirmative action in principle", which is not the same thing as being in favor of it, or being interested in expanding the scope. He's just saying he accepts the status quo at this time.
There's also various flavors of affirmative action. There's a whole spectrum from the original meaning of "take positive steps to ensure that all qualified minorities know of job openings" to "you WILL hire a minimum number of minorities, no matter how unqualified". He has side-stepped mentioning where on the spectrum he resides.