I mean, the resurgence of games with colorful art styles across the industry happened for the same reason why last gen was all about hyper-realistic brown and gray- because the people making decisions at most video game companies tend to want to follow whatever the big trends are. In the late 00s/early 10s most of the biggest games not made by Nintendo were done in very realistic settings and art styles- CoD, GTA, Assassin's Creed, etc. These days the games making the big money are all done in much more colorful/cartoon-ey art styles- League of Legends, Clash of Clans, Minecraft, and now Overwatch, so that's the trend everyone's going to be chasing after. In another 10 years or so people will start getting sick of colorful cartoon-ey shit, a new mega-franchise or two will come around in a realistic art style, and the cycle will begin anew.
I mean, there are other factors at play too, of course- the two categories in question target somewhat different audiences, technology/budget is a factor (realistic-looking games became popular after tech got to the point where games could actually kinda look real, the bigger "cartoon-ey" games were made on lower budgets and/or designed to run on phones/cheap PCs,) and so on. But in the end it's mostly a few people starting new trends via doing their own thing and a lot more people chasing said trends.
