Clerics & Thiefs Thac0 and HP progression

This is something I never understood about the AD&D rules (if BG uses them correctly).

Why has the Cleric better Thac0 and HP progression than the Tief ?

The Cleric:

Pro:
- Cast Spells (lots of strong buffs making him even a better fighter)
- Can wear heavy Armor


Con:
- Cannot use Blade weapons




The Tief:
- Can backstab (as soon as hide & co is high enough to use it well)
- Has Traps (1 per 5 level's, powertrapping aside, because cheese, [needs to be leveled as well])
- Find traps and open locks (nice to have, but traps are not that bad, and for important chests knock spell does it as well)
- Detected Illusions (nice to have, if you can speare the points)

Con:
- limited weapon selection too
- Only light Armors


So why has the Thief even the lower Thac0 and HP progression ?
I think it would make more sense to let the Thief have the better Thac0 and HP gain right ?

Or is there something I overlooked. For me it even makes more sense from the ruleplaying part.