That's not really true. Many of the most famous RPG lines have a history of long periods between editions.
Hero is technically on its 6th edition, though only 3 have been published in the past 20 years.
Yeah, but one of those editions -- 4e -- was responsible for most of that time: 13 years. And the game was in limbo for some of that time, and we will not mention the Fuzion-powered version of Champions. 6e followed 5e after 7 years.
Champions 1e, 2e, and 3e all came out in, what? An 8 year span?
Storyteller is on its 2nd edition after 19 years
Storyteller? You mean the White Wolf system? Nah, there've been more than 2 editions. Vampire the Masquerade's 2nd edition came out like a year after 1e. 2e revised was 6 years later (edit: and got replaced with nWoD six years after that). IIRC, the other oWoD games had similar spans between new versions.
The nWoD has lasted six years so far.
Warhammer is on its 3rd edition after 25 years
[...]
Runequest just published its 5th edition after 32 years
Both were out of print for many of those years, though.
Exalted released its second edition after seven years
Five years, actually.
I think the DC Adventures license helped spur the new edition. Publishing DCA, you'd definitely want to update the rules to clean things up; and once you're updating the rules for DCA, you notice the stock on 2e books is low, it would make sense to use the new version of the rules to release a new version of M&M.
Also, the DC license might limit the number and/or type of books they can do; but if M&M and DCA are really the same system, then maybe they can lure new people in via DCA, and sell them M&M books, too.