Why daggers require Intelect?

They are melee assassins signature weapon...

Critical strikes are associated with intelligence, and daggers are associated with crits. Hence int weapons.

A dumb assassin can kill, but a smart assassin can get away with it.

Also, it takes a lot more brains to kill someone with a small dagger than it does a massive sword.

Because they can roll the spell damage mod and make good witch weapons. I'm a little hazy on this but there are two grades of daggers, the dex/int ones and pure dex. Pure dex ones don't roll the spell mod i believe, while the hybrid does.

Only one type of dagger, and they are int/dex always.

You are thinking of swords that have 2 classes, one str/dex and other pure dex.