In Gen IV, Infernape could outspeed basically everything unboosted, with the exception of a few threats, like Starmie, Jolteon, and Gengar. However, Gen V added many new threats to the metagame that outspeed and OHKO Nape, such as Lati@s, Tornadus-T, Alakazam became relevant, and the Musketeers generally do the whole "108 speed fighting type" thing better. Also fast scarfers like Keldeo, Terrakion, and Salamence are on most teams, so Infernape finds itself getting outsped and OHKO'd too often. And nobody suggest Scarf Infernape because thats a set thats been proven to be awful.