I have friends who still run 2E campaigns, a couple who still use the original brown booklets that started D&D, and quite a few who love 3E. When 5th Edition comes out, I am quite certain a percentage of 4E players will turn up their noses and insist on staying with that system. I think both players and game publishers are missing out on an opportunity here. Why can we not have an OGL that includes at least parts of earlier editions of D&D?
There's no need. The retro-clones do exactly what you ask - they used the OGL and SRD to reverse-engineer the rules for the old editions, thus creating a 'new' game with compatible rules... and thus allowing them to support those rulesets.
The key thing that's not covered are the campaign settings...
Campaign settings like Planescape and Dark Sun never felt the same when converted to 3E or 4E, how come we cannot publish articles and supplements to support the original material?
The campaign settings are valuable IP, and so WotC are (rightly) reticent to simply let people at them. These will
never be opened under the OGL or any equivalent license - although potentially they might enter the public domain in 80 years or so.
However, what
was done in early-3e days, and
might potentially be done again, was for specific third-party companies to license the settings from WotC for a time - White Wolf licensed Ravenloft and Margaret Weis Productions licensed Dragonlance. (Of course, in both cases they chose to convert the settings to 3e, because that was where the money is.)
Both those licenses ran out, and one of the parties involved elected not to renew. (IIRC, White Wolf allowed the Ravenloft license to lapse, while WotC chose not to renew Dragonlance.) As for whether WotC would be willing to enter into a new agreement for one of their settings... well, you'd need to ask them. (My suspicion is that they're not really keen... but that everything has its price.)