Does it not OHKO special-oriented pokes?
You'd have to run some damage calcs, but from what I remember it doesn't do it any better than SE coverage or STABs.
Metagross and Gigalith (lulz) are the two highest-ATK users. The Gen V writeups actually have movesets for Gigalith that use it, but at the same time, Gigalith was NU and faced less powerful physical threats, and had a crappy movepool.
For Metagross... what does it give up to run it? Coverage or a boosting/support move, all of which are better than Explosion in the long run. Not many special attackers can take more than one or two Meteor Mashes to the face to begin with.
Like I said. It comes down to power (250 BP is not really that great on a self-fainting move; STAB would be nice but only Lickilicky gets that), availability (most things that can use Explosion have mediocre Attack stats) and 4MSS (Metagross has better things to do with his time, like smashing things.)
Also important to note is that there aren't many special-oriented threats that are worth an explosion that won't also make it hard to use the Explosion in the first place. Take this matchup...
252+ Atk Metagross Explosion vs. 0 HP / 0 Def Hydreigon: 335-395 (103 - 121.5%)
A clean 1HKO. But...
+1 252+ SpA Hydreigon Fire Blast vs. 252 HP / 0 SpD Metagross: 418-494 (114.8 - 135.7%)
Specs Hydreigon gets an even cleaner 1HKO and outspeeds.
I wouldn't blow an Explosion for anything short of a threat like that, and in this matchup the only way to pull it off is on the switch. You won't get a revenge kill; your target will just switch out into something tanky that probably still won't get a 1HKO, and that's if you're lucky. If you're not, he'll switch into something like Aegislash and you'll Explode something that will sit there and laugh in your face about it.
It's entirely possible I'm wrong (there may be a new 'mon that's a great user), but as it stands, if we're talking OU-level play, there's no pokemon with the unique combination of high Attack (to make it count), high Speed (so that they can use it as a finisher rather than simply playing as a suicide bomber), and the open moveslot for something so situational.