There were earlier top down RPGs, for example Dark Sun.
Dark Sun was a death knell for SSI. Fallout 2 sold a bit over 100k copies. That was considered "good". Meanwhile Baldur's gate sold over a million. Baldur's Gate was seen as the second coming of the Gold Box (whether you agree or disagree, that was the hype at the time). No BG=no IWD, no PST, likely no Arcanum or ToEE. All those games were funded because of BG's numbers, not Fallout numbers.
Bioware makes FPS games for EA instead. Fallout devs end up at Ion Storm or making Deus Ex knockoffs. No Diablo *also* means no Divine Divinity, which means no isometric renaissance games like Div:OS or Pillars.
Without the success of BG, no chance of greenlighting all of these games. Possible some other company would have pitched a Gold Box style revival but who knows. At the time, everyone else was pushing 3D (Descent to Undermountain, Thunderscape, etc.)