DinaIsha We didn't say Imprison was bad, just that it's not always the best option, and a wasted option against teams without a specific mon. Sort of like how Moldy Pursuit is really friggin' good at killing Sheddy, but pretty much crap against everything else. (And on Sheddy, a well played Sheddy is really hard to lure out and kill. Against good players, I've had games where I spent half the match or more just trying to get their Sheddy into a killable position so my sweeper can finally come in.)
Also, it's a well known fact that Belly Drum sweepers hit really hard with their STABs. The set is still shut down by good Unaware, however, since after Belly Drum and no recovery they can Recover stall your measly 8 PP or, if using a LO, HP.
252+ Atk Life Orb Aerilate Mega Rayquaza Extreme Speed vs. 252 HP / 128+ Def Unaware Cresselia: 218-257 (49 - 57.8%) -- 96.9% chance to 2HKO
50% is a lot, but Cress can just spam Recover for 5-6 turns until Ray KOs itself. Or less if you have prior damage. You need either 2 crits, crit + hazards, or for Cress to switch into Extreme Speed to reliably KO it. And this assumes Cress is not running Leftovers or Rocky Helmet, especially if combined with Spiky Shield or (though rare) Cotton Guard. If it is, then Arceus help you since Cress can just revenge KO with Volt Switch if you're forced out to try to Belly Drum again later.
There's also your two turn set-up time and 50% HP cost to consider too. That's ample time for someone to stop the set before it gets going. Or anything that can take a hit, like Registeel or Aggron, dropping a Spore on you and then healing off the damage while forcing you out. Or KOing possibly in Aggron's case, since its Gear Grind doesn't hit too shabbily.
Now, don't get me wrong. Your Ray is a friggin' nuke that'll utterly devastate unprepared teams or those who can only answer with Imposter. You asked why that kind of thing isn't seen more often and I'm just giving you an answer.