Mega Evolutions.
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 M-Lati@s.
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.
