Originally Posted by
rs2excelsior
The ease of loading thing might well have something to do with it--the offset you see with bayonets from this period helps to keep the blade away from your hand when you're loading, and it'd be harder to get that with a blade. Loading a musket with bayonet is really no more difficult than without (though I've obviously never done it while being shot at).
The "stab and twist" method wouldn't work as well with a sword bayonet, either... although now that I think of it, that technique might have come about after the offset, with the offset initially designed to allow for easier loading. I'm honestly not sure which came first, so to speak. I'm also not sure whether a triangular blade or a sword-type blade would be easier to manufacture... it'd probably depend a lot on how you're making them.