The way I see it, it's because they expect this new generation of consoles to attract a new generation of gamers, people who may have been intimidated by Halo because of the subtitles and numbers, indicating they had to do work to catch up. In this way, it seems more inviting, like it's a reboot or the first in a new series. That's obviously not true, but they're probably advertising and tailoring the experience a little more to cater to those individuals.