Wow. Considering how I’d read about the Ariane 5 from decades ago, I thought they would’ve made it past 6 by now.
The Ariane 5 was revised 5 times (“G”, “G+”, “GS”, “ECA”, and finally, “ES”.) according to wikipedia, I guess they could have chosen to call them 6,7,8,9 and 10 
My guess is there’s a different set of certifications/processes for a update or modification to a current gen design vs a next gen version. They probably couldn’t meet a schedule unless they kept the naming similar (as odd as that sounds, it happens a lot).
Ariane 6 is a completely new rocket with new possibilities. One of the big differences is that it is able to release several payloads, each at a different altitude. It can go further in space and its last stage can go back in atmosphere to disintegrate to not become a space debris.