Remathilis
Legend
True, but it provides a framework to start from as opposed to starting from scratch.They would have to pretty much create a whole new SRD for 4e too, unless it can piggy-back on the 3e or 5e one. The 4e SRD is in a very different, and much less usable, format than the 3e and 5e SRD.
The real reason though is that I don't imagine WotC is overly quick to re-expose the "warts" of AD&D again. They can release 3e "as is" for the most part, and 4e could just be given necessary adjoining info to complete it, but AD&D would require a "ground up" rewriting of the actual rules (removing Gary and Zeb's voice from the projects) which at the very least would change the nature of the works (stuff we argue about today, like the definition of hit points or alignment) and could possibly be used as an attempt to correct problematic elements (be it as innocuous as clarification of the 1e initiative system or radical as removing female strength limits). Essentially, the SRD would require a whole new version of AD&D to be written, and that is a project with few upsides and a lot of downsides (fan reaction, media reaction, competition with OS games, etc).
So I feel the WotC editions 3, 4, 5) are a safe enough bet. They were already partially designed to handle a SRD and the work to them is lesser. AD&S Don't is a brand new project in service to a small market already primed to dislike the results. It's simply not worth it.