2 things that are really important about the spread of christianism : monotheism and afterlife (moral).
And i refuse to accept that afterlife was a reason for conversion.
Norse Religion had several ends one could take after one died:
If you died as a good warrior you went to Valhall, that is most commonly known.
If you drowned you went to some Seagod/Giants Hall (i forgot his name) and feated with him. That is if your friends threw some coins into the sea for you.
If you died as a freeman, however not as a warrior but as a worker (of old Age or Illness) you went to Thor's Hall.
If you were a woman you went to Friggs Hall if im not wrong.( could have been some other Goddess) Depending on different sources a woman either ended up serving the men in Thor's Hall or feasting together with them.
If you were an Oathbreaker, A Traitor or a Coward you went to Hel, awaiting the end of times to join with Surtrs Fire Giants on Ragnarök to kill the Einherjars.
If you were a Slave you served either in Valhall or in Thor's Hall (again depending on the source)
All in all outlook wasn't that bleak if you were a woman or a normal peasant in the field.
Besides what has Monotheism have to do with it? By that logic Asian and Indian Beliefs should have died out...
The fact that the myths present in Chrisitanity can easily be arranged into some kind of narrative and formed into a coherent book probably helped. Yes, all religions prior to Chrisitanity which it displaced had stores, but these sotries were in general stand alone events rather than one more continous narrative.
The Norse Myths were also arranged in several books. Good they were less important to the Faith itself but yeah.