Wizards of the Coast released the core rules of D&D 3.5 under the OGL, but only added a small amount of material after that point.
Was there some historical reason that the rules for psionics were released in this fashion when so little else was?
Unless a WotC board member comes strolling on to this site, it is hard to come up with an answer that isn't hearsay and speculation.
We know that the original 3.0 SRD is published in 2000, and came to include the base system of the time, as well as the significant 3.0 subsystems:
Two monsters of the 3.0 Monster Manual 2, September 1, 2002 (Razor Boar and Scorpionfolk) were released as Open Game Content as well, becoming the last WotC content relating to D&D to be released under the OGL until Unearthed Arcana came out in 2004 (but that is a special case, see below).
Given what we know of WotC's and D&D's history, we can find some factors that may be relevant in their departure from the OGL philosophy. I leave this article on WotC's history as reference.
Unearthed Arcana (2004) is another story. It was (as far as I know) never added to the official 3.5 SRD, but it did contain content originally released under the OGL in other d20 publications (like d20 Modern, the core book from 2002), so it could not be made illegal to use that content in other publications, like this handy reference page.
So. My final word is that I say I have reason to believe that psionics (unlike the other 3.5 subsystems) were included in the SRD because that system was developed during the small timeframe during which the OGL was truly being pushed forward by a Ryan Dancey-directed, OGL-positive, Pokemon-money-overflowing WotC, whereas the others... weren't.
Psionics has a very deep/rich tradition in DnD, my hypothesis was that by releasing it as OGL they were baiting the people of the "old school renaissance" to release d20 books with the OGL psionics, thus providing some free d20 publicity among the really old players market.