I have a general question:
Was there an official explanation as to why, given the vast expanse of the multiverse, the githyanki chose to live in the Astral and the githzerai chose Limbo? Their respective placements always seemed a bit random to me.
I could see some benefits of using the Astral for trade or ease of moving troops, etc. But I could see a lot of the same arguments for being based in the Etheral. The placement in Limbo seems very arbitrary to me
If there is no offical answer (or hints), did anyone create an in-house answer? (E.g. perhaps the githzerai are seeking a lost tomb/library of their leader that was hidden in the expanses of Limbo's chaos; or the githzerai believe that being able to impose their will on chaos will give them a mental/psionic edge in combat with the mind flayers or githyanki)
The Astral Plane was where they were already. The illithids effectively ruled the Astral at the height of their empire, and they brought their slaves along. It was an effective hub for their planar and prime territories, especially given the squidheads' innate ability to travel there. To this day, elder brains store the bulk of their substance in the Astral. The githyanki certainly weren't going to leave, then, and risk allowing the illithids to regain control over the one plane essential to their empire's rebirth. Controlling the Astral is their way of crippling illithid growth and potential.
The githzerai just wanted to be free, and to be left alone. Limbo is the absolute freest plane in the multiverse, and one whose inhabitants are among the most isolated. A bubble in Agathion would have been more private, but they didn't want to be prisoners, just to be left to their own devices. In Limbo, one can customize reality itself with the power of will. Most divinely morphic planes are beholden to the gods and laws of nature, but in Limbo the laws of nature can be customized by anyone who can shape chaos. Master oneself and one can master chaos, and one needn't serve any other master. Limbo is too wild for the illithids ever to conquer, let alone the githyanki. The same isn't true of the Ethereal.
The githyanki took over the Astral in imitation of their former masters, inheriting the illithid dreams of domination. The githzerai wanted no part of that.