I think the one of the biggest reasons was FF7 just came first and helped to bridge common themes in the FF universe to a more novel (at the time) style of gameplay which included 3D graphics and a significantly larger story with a more expansive world.
IMO FF8 took a lot of what made FF7 critically acclaimed and fine-tuned it while giving a more modern and realistic story/world.
I see FF7/8 as a sort of renaissance to the FF universe; FF9 and beyond owing a bit of themselves to what 7/8 accomplished.