I can't speak as much for the changes from the 90s to 10s, as much as the changes from the 60s/70s to the 80s/90s, but clearly changes in audience, art technology, and aesthetic have had an influence on anime styles.
How Anime Art Has Changed: An Explainer
Anime, you’ve changed! To be honest, the look of it is always changing. Styles and fads suit each age, so let’s see how anime has evolved over the past few decades.