CS was 3D shooter and not a DOS game - quite important distinction as it gave better(and nonpixelating) graphic. Pre 1.6 CS had many modifications already - frankly I stopped playing CS before 1.6 and had my fun.
Sure, CS was arguably the most popular and long-lasting of the FPS of that era -- but let's not pretend that that was because of technical superiority, it won out because of gameplay.