They...didn't. I'm not really sure why you think they did.

I mean, Scandinavia is still inhabited by people who can draw an uninterrupted cultural line between the events of the sagas (insofar as those are reliable) and the modern day. There was no mass migration and replacement of culture in Scandinavia that started in the late 8th century.

Now, if you're asking why víkingar raided Britain and Western Europe, well, the simple answer is 'there was lots of moveable wealth there compared to the Baltic, and it was owned by people who weren't politically relevant to them.'