There's no answer for why he's white now, and no such answer will make sense anyways. This is just one of many things that illustrates that this is a reboot. Khan is a white guy now despite whatever was said in the past or whatever seemed to make sense.
And I mean, it's obvious that they at least started to go for someone who had a darker skin tone at first in the casting process. They even went strangely far enough to mostly look exclusively at Latinos, for whatever sense that makes. At some point in the process they just said, "Fuck it, the skin color isn't important to us." All they wanted was a good actor.
They wanted to make sure that they got that, and in doing so they fulfilled the pale, deep-voiced, British white male villain cliche. They whitewashed him clean and now don't have to bother with the whole Middle Eastern guy is a terrorist thing.