<<most effective for combat/self-defense and why?>>
I'd class daggers as a subclass of knives., so I don't get the "versus" here.
A dagger is a specialized thrusting knife, thus not very versatile as a tool.
I'd take a Khukuri over dagger anytime, as it has mass for the slash and can stull thrust in a pinch. Kind of the Himalayan bowie knife, and Axe in a Knife package.
<<Why didn't the dagger show a more prominent role in frontier America? >>
A dagger is a specialized thrusting knife, thus not very versatile as a tool. A frontiersman is more likely to need a knife he can use for skinning, preparing food, chopping wood, etc.
If he had need of a weapon, he was better off with the firearm for range, using the knife (primarily a tool) as a last resort. A blackpowder rife makes an excellent and devastating club of in close, and that's what he'd do after firing the first shot.
So there you have it. My opinions thru my interptretation of historical research.
Now there are exceptions to this rule. Certain NorthWest tribes (Tlingit) made double bladed daggers for hunting. They hunted moose with these massive daggers made of native copper, while other native americans were using stone points. They would drive moose to the water, and while the moose was swimming the river or lake, they would paddle alongside in a skin canoe, and jab the moose until panic, exhaustion, or bloodloss overcame the beast. So the Tlingit dagger was more of a hand-spear, used in the icepick grip.
Barnes and Noble's book "Swords and Hilt Weapons" (various authors) has a good pic of these fabulous and rare daggers.
Keith
En Ferro Veritas