It's already said, but I put some emphasis on this:
"Service ceiling (
without external ordnance and stores), km 7"
(Sukhoi's official website
Sukhoi Company (JSC) - Airplanes - Military Aircraft - Su-25Ê - Aircraft performance)
With ordnance this ac is simply unable to climb further. I agree, (perhaps) it doesn't need to climb to 330 to fire a missile at a 330 target (I don't know the radar angle capabilities, but keeping in mind it's primary role is ground attack I really doubt it's able to track targets at a high angle), but for what reason the Ukrainians would have used a SU-25 for such dreadful mission and not a SU-27, a SU-24 or a MIG-29 instead?!