The simplest answer is that "to hit" and "to wound" have been around since the beginning, but when they decided AoS was going to be the more accessible of their two wargames, it meant removing things like WS vs WS and S vs T. I'm no math man, but I'd also wager having two separate rolls helps to even out value distribution, i.e., if we only had a single hit roll, we might see greater spikes or failures in each batch. How many times have we rolled spectacularly to hit, only to even out the score on our to wound roll, or vice versa?