Defences are not everything - Breloom is a massive offensive threat in OU who packs some one of the most powerful priority attacks in the tier, the ability to reliably put Pokemon to sleep and great coverage moves.
Mach Punch, coming off Technician and base 130 Attack, is an extremely powerful priority attack that allows it to revenge kill plenty of Pokemon with ease. Priority Fighting moves are fantastic in OU as they allow you to OHKO threats like Bisharp and Scarftar. If you run Life Orb you can also KO Excadrill, Lopunny and many others. Bullet Seed is also insanely powerful; to put it into perspective, if it hits every time, it's almost as powerful as Mach Punch is on several Pokemon weak to Fighting. If you use the Life Orb set, even physically defensive Rotom-W dies to it. Rock Tomb is a fantastic move due to its ability to catch Flying mons on the switch and lower their Speed to hit them again, if not KO them. Breloom can also run Technician-boosted Force Palm or Swords Dance in its place, though Rock Tomb is better in my experience.
Breloom also packs the ability to put Pokemon to sleep, and as you'd know, this is incredibly useful as it can allow you to put a switch-in to sleep and hopefully kill it or switch. Breloom's Speed isn't fantastic, but it puts it into a great speed tier. It bumps it over the amount needed to outspeed Azumarill and Magnezone and allows it to move before base 80 Pokemon that don't run a Speed-boosting nature, which means it can move before a ton of Pokemon who'd otherwise kill Breloom before it does anything.
Breloom can also run a Poison Heal set that runs bulk - you're right, Breloom doesn't have great bulk and typically runs Focus Sash as a result, but it does have a decent defensive typing and a passable 80 Defence (same as Alomomola which is bulky as heck so it's not that bad). It's not as viable as it was in BW before the rise of birdspam, but it can still function as a decent physical wall with sleep support.
tl;dr: it's far too powerful for UU, and it is one of the few Pokemon with access to accurate sleep moves. Fantastic Pokemon in OU, basically zero chance of falling to UU.