Hi, I have a master in religious studies but this isn't really something I have studied deeply.
However, you have to understand that "religion" is a very vague and limited term that does not really have a clear definition, and whether it is or isn't used to describe a given phenomenon is more a matter of power and legitimacy than of analysis. "Religion" always functioned in contrast with other, lesser things, such as "superstition" or "magic". In the colonial era, Europeans had "religion", and if the people they interacted with had comparable things (such as a sacred text), they too had "religion" -- but if your traditions were orally transmitted, for example, you had "primitive superstition".
Now on to Christian religion / Norse mythology. "Norse religion" is as sayable as "Christian religion", but mythology specifically refers to a body of myths, i.e., narratives. A Norse sacrifice to Odin would be "Norse religion", but it would not be Norse mythology.
(EDIT: and there's a lot of grey area here. Are fairy tales mythology? They concern spirit creatures like gnomes and fairies... Is a story only mythology if it is about a god? Then, what's a god?)
However, you can already see the workings of power and legitimacy in this comparison - while we may speak of Norse religion for a Norse ritual, it would offend many to speak of "Christian mythology" for a Bible story.
Words like "Greek mythology" or "Norse mythology" also generally imply far more cohesion and stasis than the Greek and Norse contexts actually had. For Greece we know more than Norse religion, but Greek myths have incredible variety and dynamism throughout different area's and time periods in Greece. Gods and stories about them would constantly change. For example, Aphrodite is not originally a Greek goddess -- she was a Phoenician goddess "imported" by Phoenician traders, who the Greeks adopted into their system. (This "adopting" of different gods, which is utterly alien to the modern day concept of "religion", also points to how different they thought about gods and worship compared to modern day Christians and modern day post-Christian seculars). The idea of "Greek mythology" is very much a modern day edited collection that erases the multiplicity of actual ancient Greek religion.
TL;DR comparing "Christian religion" and "Norse mythology" is a bit like comparing apples and oranges; or more fittingly, comparing apple trees with oranges.
Tl; dr: I suppose that the employment of the different terms (mythology/ religion) primarily ascribed either to the focus on the work or at least partly to the nature and type of primary written evidence.
I hope that the following previous threads should be the departure point for exploring OP's question:
Both words, "religion" and "myth(ology)" have certain ranges of definition, but at least well-known alleged Old Norse texts on "the gods (æsir)" are generally not so straightforward to reconstruct what Old Norse-speaking people practiced every day before their acceptance of Christianity.
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(Adds): /u/Steelcan909/ also illustrates the basic information on what we know/ don't (can't) know about "Old Norse religion" based on the extant evidence and state of research below:
I also forgot to remark that some (especially) academic books have focused rather on "Old Norse" religion than on mythology for long, especially such as:
/u/Platypuskeeper has previously answered How much do we actually know about the ancient religious traditions, deities and narratives of the Norse?
/u/bloodswan has previously answered Other than Snorri, where do we get our current understanding of Norse mythology from? as well as How do we know that Snorri Sturlson didn't fabricate Norse mythology?
/u/Steelcan909 has previously answered What mythology would a 800-900 CE person living in what is now England believe in?