Mass slavery of the Roman form wasn't "popular" among the West Germanic tribes because they had their own system of subjugation of a more feudal model, or at least something transitional between the classic Germanic class system (king, nobility karl/jarl, freemen, laymen) and what became feudalism.

Or rather, after their brutal occupation and displacement of the Romano-Britons they came to emulate the Franks (the most successful post-Roman polity in Europe) in the structure of their economy. Think Serfs rather than Slaves.

Slaves in the Roman system could be freed and become Roman citizens. In fact it was rather common after a generation or two. Peasants were peasants forever. Not saying one is more just than the other, but Christian influence in Europe certainly hadn't made for a more just system. Just a more theologically based one (God chose your King and Lord)