also say that the large number of illegal aliens also affects such statistics negatively, but does not want to offend certain voters
I don't know why you would think that would be offensive when it's just common sense.
Let's say that's my theory why Perry does not use that argument. He is afraid of offending Hispanics. I suppose it is possible that he has never thought of that as an explanation of why Texas compares worse statistically to other states in some categories, but that would not be a good sign.