I think it's both the effect and the cause. Even personally I'd rather use a program that looks nice as compared to one that does a little bit more but is ugly. So better looking ones will have larger audiences, and thus to get a large audience you need to invest in looks.

In terms of simplicity a lot has to do with the capabilities of the hardware. The computers were much more limited, so the ceiling was lower as compared to today.

Example would be something like making a game. When I started coding, having just text on screen was far from being the best, but it was not totally out of place. These days 3D or nice looking pixel art is basically the baseline.