Why is the Solar separate instance from Waking Sands / Rising Stone?

[Enkidoh]

cause zone would probably crash on patchday / expansion launches

This is the exact reason - back when ARR was in Beta/launched, the Solar was actually indeed just part of the rest of the Waking Sands that everyone could freely enter at the same time... which ended up as well as you would expect. It became as crowded as Grand Central Terminal in rush hour for months afterwards, it was so bad at times it was outright impossible to target Minfilia or any other npc while in there half the time due to the huge throng of players clustered around her, to say nothing of all attempts at immersion going out the window when a room that is meant to be mostly empty save for yourself and Minfilia...is crowded with adventurers.

So after a few months or so SE redesigned it in a patch so that the Solar was changed to become a seperate instanced room from the rest of the Waking Sands that only the player themself could enter, and this has continued with the move to the Rising Stones (and the Intercessory in Camp Dragonhead - Haucherfaunt's so-called 'Falling Snows') also being instanced too.

It's not too much more time spent loading into it really, it's the price that is paid to ensure smooth gameplay.