Bungie engineer Roger Wolfson, says this is because better tech can influence gameplay factors in the shooter genre.
"I'll speak for the hypothetical player," Wolfson told Digital Trends. "I have a disadvantage sniping across the map because [my opponent with a next-gen console] is only two pixels on my screen and I'm four pixels on his. You see that in the world of PC gaming, where people are always racing to the best video card to give themselves the advantage."
"Regardless of where the reality is, there's definitely a perception among gamers that better hardware means you have an advantage. We don't want to have to enter that fray, so to create the best, most level playing field, both actually and perceptually, we separated it by platform."