It is a bit lame, but I think the only way to answer this is with specific causes for each province--there was no real overarching narrative that explains them all. For example, France, Spain and Italy were conquered by Germanic groups that were relatively "Romanized", by which I mean they strove to integrate themselves into the areas they conquered, and as importantly, the socio-political structures within the provinces were still functioning when they were taken over by the Germans. Britain on the other hand had suffered a severe collapse before the coming of the Anglo-Saxons, who were anyways not very "Romanized" at all. Romania is a weird case, and if there has been a good explanation for it, I don't know. And North Africa of course was conquered by the Arab Muslim expansion, and so speaks Arabic.

The post-Roman period in the West is really one in which the many provinces go there own way, and so there isn't a unified narrative that accounts for all the variation.