It's Fighting-type, learned by fewer Pokemon, and has 5 PP, rather than 10. So in other words, it's a crappier version of Protect.
My guess would be that, early in development, Game Freak planned to make attacks like, Feint and Shadow Force, which could hit through Protect. The original purpose for Detect might've been for it to not to be hit by these attacks, and be the "Ultimate Protection."
Purely a guess, but it seems like the only logical reason behind the existence of such an attack.
Maybe it was also added to make it seem like Fighting-types also had some tactical/Defense options as well.