I think there's a couple of reasons.
For one money is tight for a lot of people these days, I know it is for me and gaming only gets more expensive as time goes on, I've gone from owning a PS2, Xbox and Gamecube to a PS3, Wii and 360 to now only having a PS4 and a PC, I decided to invest the most money in a PC since the vast majority of new releases come out on PC anyway and it's usually the best version with the best graphics and so on, so it's no surprise that consoles would decline in popularity when they're no longer the best option.
Then you have kerfluffles like Gamergate which damaged the mainstream's perception of gaming and gamers, it's no surprise to me that 2010 would be the peak year as that was the peak time period of gaming's mainstream popularity with games like Guitar Hero and the Wii, now unfortunately gamers are back in the doghouse as being as seen as a seedy, disreputable culture, for a lot of people today "gamer" is probably synonymous with "Alt-Right" (not saying this is at all correct, just what the mainstream's perception is), I mean there was an episode of Law & Order depicting gamers as a pseudo ISIS like group, the days of everyone and their grandma playing Guitar Hero and the Wii are long gone.
I've also heard of weird stuff like kids today preferring to watch people play games on Youtube rather than play games themselves.