As for the "relative" abundance of Old Norse texts, while much more can always be said, I hope the following previous post of mine might satisfy OP's curiosity at least to some extent:

• Why is Norse mythology so well-known compared to other contemporaneous Germanic religions?: together with /u/Platypuskeeper
• Why are most sources of Norse mythology from Iceland? : together with /u/sagathain
• How knowledgeable and respected was the average village priest during the 1300s?

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In short, the survival of vernacular literacy (Old Norse) was primary due to the integration of Old Norse literature (especially) poetry as a part of formal (grammatical) education as a source of social prestige in medieval Iceland, and this dual (Latin-vernacular) literacy among the elite clergy might have had an origin in Anglo-Saxon church (monasticism).

Concerning non-mythical (that is to say, not related to pagan= Christian) texts, some Christian prayers written in Old High German (Ahd.= Althochdeutsch) are certainly extant, but not so much. Relatively low prestige of the vernacular Germanic in early medieval continental church where scribes worked might have determined this regional difference, though German scribes at least writes Heliand, life of Jesus in verse form, in the monastery.

Add. Reference:

• Patzuk-Russel, Ryder. The Development of Education in Medieval Iceland. Berlin: de Gruyter, 2021.