The smooth scrolling was easy, the EGA had more or less the same hardware capabilities as a C64 in terms of smooth scrolling. It's just that few or no games had tried to use it. For VGA, Mode X was the real game changer because it allowed to use both double buffering and smooth scrolling at the same time. A later 2D platform game that took Keen's smooth scrolling to the next level is Jazz Jackrabbit.

But as CPUs got more powerful, Carmack recognized that you didn't need hardware help anymore and that removed constraints on what games he could program. Keen to Wolf3D to Doom is when id games went from good to wow to insane.

The graphics bus was also a huge constraint. Using dirty triangles to bit bang is effective but tedious.