I guess my question would rather be, why not have many different versions being available and played at the same time. Look at how many versions of D & D exist and are being played, including the original series of books which has just been re-released. I am looking forward to setting up a campaign with my likely future son-in-law and his father this summer, along with probably in exchange joining Kevin's (the father) Traveller 5 game. (Bullet biting here.) Then you have the Rolemaster fantasy game, which we might start a campaign in, and also I still have all of my Middle Earth Roleplaying material from Iron Crown Enterprises. Each version of the game has is good and bad points, which appeal to different people in different ways.
As of last year, there were groups commenting in the D&D Next surveys playing every then published edition, and the major subversions. Mike Mearls commented on how surprised he was that some people were still playing woodgrain without the supplements.
There are 10 major editions of D&D... (OE, Holmes, Moldvay, Mentzer, Alston, AD&D 1E, AD&D 2E, D&D 3E, D&D 4E, D&D Next) and 5 of them have multiple sub-editions. (OE, AD&D 1E, AD&D 2E, 3E, 4E).
OE is very different if you lack/don't use the supplements, and/or use chainmail as the combat system.
AD&D 1E is significantly different if you use Unearthed Arcana and/or the Survival Guides and/or Oriental Adventures.
AD&D 2E has the "Player's Option" series - which includes moving to 12 attributes, point building characters, and having to buy most of the race and class powers.
D&D 3 has 3.0 and 3.5, and some would also say "Pathfinder"
D&D 4 has 4.0 and Essentials.
And that doesn't count EPT 1E (which was a close OE+S1 variant), nor the SW d20/SWSE rules, nor the 4E derived Gamma World... Nor the zillion retroclones and pseudoclones.