I'm going to attempt to answer, but bear with me a little - this is my first time answering a post on here. First thing to take into consideration would be the massive movement towards nationalism during the 19th century. With the recent creation of a new German state, many Germans were searching for a new national identity. Some, including Wagner, found it in the legacies and history and mythology of the Scandinavian Vikings, who were somewhat Germanic in ethnicity - though it's interesting that such a nationalist atmosphere was constructed around a group/time period that wasn't inherently German.

By creating a "German" Opera about a "German" legend, Wagner was helping the German people find a nationalist centre of gravity where previously there had been very little.

The push towards Norse mythology also has roots in the intense ethnic conflicts that were occurring at this time. More and more ethnicities were beginning to assert their own voices, and the Germans are a prime example of this. Having a blonde-haired, axe-toting hero to watch in the Opera and look up to was a huge part of the German mindset, and Hitler himself was an immense fan of Wagner and used his art in order to further his own political aims.

In sum, Norse and Viking traditions and mythology became highly popular in the mid 19th century because of the highly nationalistic movements coursing through Europe, primarily that of Germany. Composers like Wagner utilised Germanic (Norse) traditions to provide a cultural framework around which the German people could build a more cohesive national identity. Wrapped up in this were the racial prejudices of the time, as the Germans attempted to separate themselves culturally and ethnically from the rest of Europe.