Stat...istically
Dex is a better stat than str for the most part.
It improves a save, your AC, your initative, your ranged attack roll.
With one feat, weapon finesse, my dex fighter can now attack as well as the str fighter, but I also have a better AC and importantly....touch AC, I will go first more often, my bow doesn't actually blow, and my reflex save is actually decent. Granted, they will not be doing the damage. But that's a pretty good tradeoff, and frankly I think its worth a feat.
But if you allow dex to add to damage as well, all of a sudden my dex fighter is blowing away your str fighter. I hit and damage as well as you do, but gain all the other advantages.
And that's something a human fighter 1 could do potentially and still have 1 more feat.

It improves a save, your AC, your initative, your ranged attack roll.
With one feat, weapon finesse, my dex fighter can now attack as well as the str fighter, but I also have a better AC and importantly....touch AC, I will go first more often, my bow doesn't actually blow, and my reflex save is actually decent. Granted, they will not be doing the damage. But that's a pretty good tradeoff, and frankly I think its worth a feat.
But if you allow dex to add to damage as well, all of a sudden my dex fighter is blowing away your str fighter. I hit and damage as well as you do, but gain all the other advantages.
And that's something a human fighter 1 could do potentially and still have 1 more feat.