Because the "projectile" used is much, much larger and more massive. It's like having an elephant hit you at 55 MPH as opposed to a grenade chucked at you--the grenade is explosive, but that's more than made up for by the elephant's bulk, which will surely do much more damage. Similarly, slamming a starship (elephant) into a target will do a lot more damage than conventional weapons (grenades).
Also, in some cases a captain will use a ramming maneuver when he knows his ship's headed for a warp core breach--slamming into the target will make the core go boom right then and there if it wouldn't have already gone off at quite that moment, and that is a HUGE explosion. Extending the aforementioned analogy, it's like getting hit with an elephant at 55 MPH that's packed with TNT.