In very short answer: Shortage of the sources. Norse mythology has 'relatively' abundant written (textual) sources compared with next to none reliable for continental German or very fragmentary Anglo-Saxon one, though recent scholars increasingly became wary of how extant Scandinavian sources could be representative of the actual beliefs among the Norse people during the Viking Age. As for this point, I'd recommend /u/Platypuskeeper 's excellent previous posts in this thread or in that one.
[Added]: As for why Norse people (especially Icelanders) record some pre-Christian myths long after Christianization, see also my post or that of /u/Platypuskeeper in Why did Snorri write the Prose Edda?
I just make a brief note below, however, on how little we know about non-Norse 'Germanic' deities: Old Saxon Baptismal Bow (ca. 800) names three deities of the Saxons that the Franks required them to abandon:
'End ec forsacho allum dioboles uuercum and uuordum, Thunær ende Uuôden ende Saxnôte ende allum thêm unholdum thê hira genôtas sint.'
'(Very Rough English trans.)...And I forsake all devils' acts and words, namely of Donar, Woden, Saxnôt, and all of their fellow demons......'
Taken from: Indiculus superstitionum et paganiarum, in MGH 1 (1835), Capitularia regum Francorum, ed. Gerg H. Pertz.
We usually assume that these three deities mentioned above in the vow had been occupied the central place in 'Germanic Pantheon', but how much we actually have the evidence about themselves, not their alleged Norse counterparts? Almost nothing contemporary and reliable.
Another 'important' source on pre-Christian deities is (1st and 2nd) Merseburg Charm(s), recorded in the 10th century Old High German manuscript, in which Wodan certainly appears again, but......:
'(Very Rough English trans.) Puol and UUodan (Wodan) rode into the forest, where the foal of Balder dislocated its leg. Then, Sinthgunt and her sister, Sunna, spoke to the foal (?); Friia and her sister, Uolla, also spoke to the foal (?); then, Uuodan spoke to the foal as well, and since he was powerful, so dislocated bones, blood and joints are now tied, a bone to another bone, blood to other blood, a joint to another joint, thus it healed' (Cf. Krause 2010: 180).
That's all the 2nd charm, not excerpt. The supernatural beings invoked in the text, especially UUodan had some power to heal, and many of female 'deities' were related. While Friia seems to be Saxon variants of Old Norse deity Frigg, but to what extent they shared the similar lore? We almost don't know at all.
Note that these two are the most important and almost all the possible accounts of the beliefs among pre-Christian Continental Saxons and their supernatural beings. They are so fragmentary, however, as shown above, that it would be almost impossible to reconstruct the 'myths' solely based on them.
References:
Religion vs Mythology
Well for starters I think one ought to separate 'religion' and 'mythology' a bit here (if only to bring them back together). What I mean is that on one hand we have the actual practiced popular cult. The actual rituals that were performed, such as how people petitioned the gods for things, how animal, human and other sacrifices were made, the burial customs, the temples, sacred trees, bog sacrifices, and various customs that are only hinted at, such as ceremonial drinking and eating on graves. We really know very little about these things, and what we know is largely from archaeological records and a few hints from written sources. (Ibn Fadlan, Adam of Bremen, etc)
The Myths
On the other hand we have the 'mythology', by which I mean the stories about the gods in the Eddas and other bits about them mentioned in the Sagas. In other words, mainly the skaldic poems/songs/kveði that we have about the gods. The oldest we have of those are now thought to date from about the 9th or 10th century, but skaldic poetry continued to be composed until the early 13th century when the Eddas were written down by Snorri Sturluson in Iceland. So this is well into the Christian period (Iceland having converted or at least decided to convert in the year 1000) Snorri had many motivations for writing it down (See also answers in 'Why did Snorri write the Prose Edda?') but the reason Snorri gave was that he wanted to preserve this literary tradition. Perhaps because continental literary trends like the Chanson de Geste were catching on. But he was also elevating the Norse gods culturally by equating them with the Greco-Roman ones. Who were fairly popular literary topics at the time, in the sense that the 'Matter of Troy', poetry about the Trojan War was. But although one can write about these gods as old in a literary setting, a good Christian shouldn't believe in them of course. (And Snorri said as much in Skáldskaparmál too)
There are two Eddas, the "Elder" or "Poetic" Edda, which is a compilation of older skaldic verse, and the "Younger" or "Snorri's" or "Prose" Edda, which is Snorri's own writing in prose about the Gods, interspersed with fragments of poetry both older and of his own composition. Much of what Snorri writes in the Prose Edda are clearly things he's learned/inferred from the Poetic one, so they are not independent works.
The Eddas don't tell us a whole lot about the former topic, the religion. There's no reason to expect that either, as they were clearly not written for that purpose. Even if he'd wanted to, Snorri may not have had much to tell about pagan practices. By his era it's implausible he witnessed any major pagan practices. Whatever he knew of the 'old custom' would've been whatever oral histories of the 'old custom' existed, and the now-Christian Icelanders had little motive to portray it positively. The story Völsa þáttr, written down in the late 14th century, depicts a pagan family as worshiping a mummified horse penis, until an incognito Saint Olaf comes along, tosses it in the fire and converts them. This story is one of those few scraps that depicts an actual cult practice. There may be a kernel of truth to it (we know horses were sacred) but it seems more likely to be an caricature of pagan practices. Or what they, by the 14th century, were imagining their former practices to have been.
So when you get your average popular book of Norse Mythology, what you're learning is like 95% the Eddas. Often they have inferences, guesses and extrapolations from the text (which are often reasonable, sometimes not, but in any case not literally there in the original) They'll add bits from archaeology and other sources where it makes sense to do so, like showing the Altuna runestone in connection with Thor's fishing. It depends on the book but if they aim to present a consistent narrative they'll have to make some editorial choices because the stories aren't consistent. For instance, there circumstances of Loki's capture and punishment are quite different between Lokasenna in the Poetic Edda and Gylfaginning in the Prose Edda. But in any case this is the stuff that's popular.
We don't have 'Eddas' for peninsular/continental Scandinavia. We know that at least some of these stories were still known there in Snorri's day since we have art with Eddic motifs, such as the baptismal font from Norum depicting Gunnar in the snake pit in the Völsunga Saga. (the text says 'suæn kærþe' - 'Sven made [me]') But it was only Iceland that had this strong literary tradition where they wrote down not just the Eddas but a great many Sagas as well.
What the two have to do with each other
Anyway, so. The gods are the gods and it's not as if the Eddic stories had nothing to do with the popular cult. But it's hard to say how much they do have to do with each other. I mean, a Christmas story starring Santa Claus obviously has something to do with Christianity, but it doesn't necessarily tell you much about the religion, and may well be enjoyed by non-Christians in the way that Christian Scandinavians continued to enjoy Old Norse stories.
On the other hand, there's no doubt that for instance Thor was a thunder god and widely venerated (indeed a personification in the sense that Thor means 'thunder'. In fact 'thunder' has a common origin with 'Thor') But was this because he was a giant-killing badass like in the Eddas, or was it because as a weather god he could give you good harvests if your worshiped him, as the chronicler Adam of Bremen wrote? That's the harder question.
It's noteworthy that the popularity of gods in place-names doesn't match their prominence in the Eddas at all. Freyr and particularly Ullr are much more prominent in place names than in the stories, while popular story figures like Heimdallr and Loki have zero place names.
So the point is, we'd know very little about the actual religion. Although the Eddas don't tell us a whole lot, we'd still know a lot less without them. But we cannot take them to represent a single religious tradition either, because there probably wasn't one. The distribution of religious place names is inhomogenous in Scandinavia (no place names after Ullr in Denmark). There was huge variety in burial practices across Scandinavia, particularly among ordinary people, while for instance ship-burials for big-shots were similar across Scandinavia. Which is yet another aspect, namely class and to what extent what we have is the upper-classes' version of the religion. (who were likely more interested in warriors than harvests)
Other Germanic religion
When you go to the rest of the Germanic sphere, the situation is akin to what Scandinavia had been without the Eddas. That is, what we know of the religion or the versions of the religion practiced there, is mainly restricted to archaeological evidence and a small handful of written sources mentioning various practices, such as the Life of Saint Bonifatius explaining about a sacred tree which he chopped down.
So that's the answer to the question, and it's really rather short: What people are interested in when they're interested in 'Norse mythology' is usually not so much figuring out the nature of sacrifices, or burial practices, or other cult aspects. They're interested in the Eddas and the stories in them, in much the same way that Snorri and his contemporaries were. That is, not because they had any religious significance to them but simply because they were good stories. But for the continental Germanic peoples - or really any Germanic peoples outside the Icelanders - there is no equivalent body of text. It's a fragmentary picture that's not very satisfying as it leaves you with far more questions than answers.
