Oh this can be explained quiet well.
Isolation from theological rivals and lack of infrastructure: the romans for example already had a culture of theological debates priour to christianity (ciceros de natura deosum for example) due to their excellent trading networks to other cultures and riches that allowed them to entertain a philosophically active upperclass that wrote their shit down due to enough freetime.
This continued in the late antique were polytheists and christians were equally powerful and had to use arguments against each other.
But even then, the amounts of surviving roman theology are exeedingly sparse due to the rareness and brittleness of paper in general and a good amount of purges.
The extensive works of the polytheist theologe celsus are for example only known by a christian commentary's quotes against him.
Another example would be the arab pagans, these had trade and literacy by infrastructure but no theological opposition as their monotheist neighboors the jews sticked to themselves thus the idolaters never had to justifie their position in an equal standoff on argumentative paper but by hymns and prayers alone along the fact that they reigned supreme.
As muhammed rose to power so quick, the only counter argument the muslims recorded from their vanquished foes were appeals to their ancestors "your fathers and their fathers did it like this! ;_;" along the failed attempt to simply put the gods beneath allah and pray to both as all religious conflicts were pacified before.
Back to the germanics.
These neither had much trade going on so they rarely if ever got in contact with nonpolys before nor could they support the infrastructure to sustain a thinking class that wrote stuff down aside from the occasional poem such as the hildebrand song.
So word of mouth sufficed to keep the religion in the same way ancient people rarely got the idea to write dictionarys about their own language.
As christians came their persuation skills honed by centurys of debate with roman pagans basically steamrolled the natives, albeit war and many centurys of penalisation was needed to actually end the practizes.
>the weak
Funnily the amount of pagan shit that got injected into christianity speaks against the notion that the old beliefs were utterly inferiour.
Now, counter question:
Why is there not much secular/scientific debate between 700ad and 11th century in northern europe from christian sources?
Yeah, they lacked the infrastructure and needed cloisters of permavirgins to copy bibles all their life to at least produce scripture themselves, its not easy if youre not at the mediterran sea and the muzzies just capped the trade to the orient..