Quite simply, the addition of Mega Evolutions this generation means that Forretress' Defenses simply aren't good enough for the high Attack stats of the latest addition to the meta-game.
With Physical Sweepers like M-Lopunny, and M-Metagross and Megazard X dominating the ranks of OU, a Forretrees simply doesn't cut it anymore. Only Shuckle, M-Steelix and M-Slowbro can hope to actually stay alive.
Apart from that, the Special Type sweepers also got a boost, as seen in the almost OP Drought Powered Megazard Y with that massive Special Attack, and the ever effective [email protected]
Thus, many Pokemon that were previously quite powerful got pushed to the ranks of UU, RU, and NU.
Additionally, the introduction of some set-up moves that Forretress doesn't have access to like Sticky Web further make it worthless now-a-days.