Salamence is MUCH bulkier than Infernape. Higher in all three stats, and has Intimidate to boot. Not to mention the MANY good resists. Salamence, with some investment, can easily soak up a wide variety of attacks for 50% health or under. This is why Roost Salamence is good. Infernape struggles to take any attack for under 50%, and thus cannot properly use Slack Off. The way Roost Salamence works is: switch in on something it takes next to no damage from and force it out, throw down a powerful attack or a Roost on the switch, and then play the match from there. Infernape has almost no attacks it can properly switch into, can't threaten very powerful attacks (aside from super effectives), and is WAY too easy to force out. Slack Off Infernape just doesn't cut it.
I could be wrong, though. Feel free to disprove my argument.