[alanfang]

Not jumping for free throws isn't just a tradition. Jump shots sacrifice accuracy to make it more difficult for defensive players to block your shot. There's no risk of your shot being blocked for a free throw so players don't jump.

[philwelch]

If there's no risk of your shot being blocked for a free throw there's no reason to shoot overhand, either.

[eropple]

It's an easier motion for most people who've become accustomed to shooting that way, though.