Lots of reasons, but let's clarify a major misconception here first:

Wasn't their entire economy and political system based on seafaring conquests?

Absolutely not. Víkingar contributed nothing to the national (or, more accurately, proto-national) economy during their raiding, as raids were undertaken to increase personal wealth. The vast majority of the economic system, such as it was, was derived from traditional sources, specifically trading fur, fish, and amber.

As far as why they gave up on North America, the Return on Investment wasn't great enough to really warrant another settlement further west from an already small population. You have to remember that it wasn't Norsemen who settled Vínland, but rather Greenlanders and Icelanders. The Norwegian crown (and the Danish crown, and the Swedish crown) had nothing to do with the settlement there.

There just wasn't enough in Vínland to warrant the cost in both manpower and wealth to set up a colony there.

This fascinates me as an icelander, might i perhaps ask a sidequestion? What do we know of the natives in Vínland? are there any ruins or remains of cities in the area? Do we have any idea how many natives the settlers would have had to "subjugate?"