The question is pretty much in the thread title. It seems to me like they could’ve avoided having to educate everyone about the meaning of a new rating system by just using the one that everyone is familiar with. Were there IP issues? Was it just a case of not-invented-here? Or was it something else?
Except for the addition of eC, they seem to line up:
[li]G = E[/li][li]PG = E10+[/li][li]PG-13 = T[/li][li]R = M[/li][li]NC-17 = AO[/li][/ul]Considering it is a different organization, it makes sense that the ESRB ratings have different names.
Edit: Sorry, I just realized I misunderstood your question. You are taking it for granted that the ratings line up, but are asking why they used unique names instead of reusing the MPAA names. Wiki offers this: