Why Are Kobolds So Slow?

My players had a bad run-in with some kobolds tonight.  The party threw some flaming oil into a room where the kobolds were, killing two and prompting a failed morale roll.  The kobolds retreated out of the room and set up an ambush down the hall.  The players spent some time hemming and hawing before over-extending and rushing down the hall, causing the fighter to get surrounded and dropped by kobold spearmen.  It looked like the situation might spiral out of control but the party won initiative, allowing them to get the critical sleep off.

I hadn't realize just how slow kobolds are until I was retreating down the hall with them.  They're as slow as a man in plate but they only have AC 12!  It's kind of remarkable.

Goblins are also quite slow, at 60' speed.

One explanation, the in-world explanation, is that they're short and have stubby little legs.  But halflings get 90' speed in the monster entry (and 120' as PCs)...

I'm thinking maybe there's a game-reason.  Kobolds and goblins are the weakest and first humanoid monsters most players meet.  They're the ones players are most likely to over-extend into.  Their lack of speed limits their ability to pursue and punish player over-extensions - they might get the fighter and the cleric, but the MU and the thief might actually be able to outrun them if things go bad.

Orcs, on the other hand, have 120' speed - a big step up.  This might be a bigger deal than the full 1 HD in making them more threatening to low-level parties than goblins.  When you start meeting orcs, the training wheels are off and they can give a good chase even if you're lightly-encumbered.

It's kind of an interesting alternate lens on differentiating weak humanoids from my previous approach - maybe the significant mechanical differences are already there, and I've just overlooked them.