Garchomp is uber because Dragon-types in general are ridiculously overpowered. The offensive value Dragon as a type holds is unprecidented; ONE TYPE RESISTS IT. And 90% of Dragons can pack both Flamethrower/Fire Blast AND Earthquake to get around that, meaning that regardless of what ability your Bronzong picks, it's getting owned.
It doesn't help Garchomp's case when not only is one of those anti-Steel types (Ground) ITS STAB, but coming off base 130 Attack. Not only that, but it has 102 base Speed and 108/95/85, which is no joke. It can live most Ice attacks if holding a Yache Berry.
Its STAB moves are also far superior in accuracy, power or all-around availability when compared to Heatran's. Outrage, STAB Earthquake...that's just the tip of the iceberg, seriously.
Heatran, meanwhile, while it's legendary and has 600 BST (lol Celebi, Cresselia, Dragonite, Jirachi, Latias, Metagross, Salamence and Tyranitar? lolololol), is extremely predictable. The vast majority of them run Fire Blast / Earth Power / Explosion / HP Ice.
It's also much slower and doesn't 6-0 teams like Garchomp can. It has utility in other uses, but its STAB attacks are much easier to absorb. Fire resistance is necessary for any teams using Scizor (which...well, half of them do), so a lot of teams prepare for it.
Garchomp was the reason people used Bronzong and Cresselia. Notice how nobody does anymore? That's because their defenses were only useful to live boosted Outrages.
Heatran isn't enough of an immediate threat to people to warrant an entire change in the metagame. Arguably, it's the Cresselia/Bronzong of this "format" of the metagame, since so many goddamn people use Scizor and Heatran can own it so well. (4x Bullet Punch resist vs. 4x Fire weakness...so yeah.)
Garchomp is a monstrous sweeper. Heatran is the metagame's antithesis. Their uses are totally different.
You're only classified Uber if Standard can't handle you. A slow, 4x Ground-weak Pokemon is EASILY disposed of. Garchomp, when it was OU...wasn't.