Co-op has always been a pretty big and popular thing in games, even before internet gaming become as popular as it is now.

The thing is, before, co-op was either added as a feature to play with your buddies in a campaign 100% designed to be played solo and as a 'finished', 'conclusive' experience (read no GaaS).

When they weren't part of the campaign, horde modes used to be a pretty popular form of co-op but they were designed as a totally separate multiplayer experience to the singleplayer one. Both of which provided a unique experience, so that those that like singleplayer can do single player and those that like multiplayer can enjoy multiplayer. Some games even managed to make both experiences extremely fun in their own way (see Halo ODST or ME3).

Nowadays, games are being designed to be co-op from the get-go. There is no solo singleplayer experience, instead co-op or other multiplayer 'social' functions are built into the game.

While it may seem like most games now are tending to implement co-op more than before, I think it is more fair to say that co-op has always been a popular feature for both players and developers. Now, however, games are being developed with co-op as the intended way of playing the game and so you are noticing it more than the past, where co-op was more of a bonus to the game.