Well, to be fair, they only did it after the Dark Elves' entire homeland got devastated, first after Lie Rock finally resumed its collision course with Vivec, and then when Red Mountain erupted. Until then, they were pretty bad at keeping slavers from raiding their homeland, despite it being a treacherous swamp with a terrain that alone should do the job at keeping out foreign invaders. The fact that slavers raided them at all does paint the Argonians as pretty pathetic. Their invasion of Morrowind was less of a badass move and more of a "kick them while they're down" play.

On a somewhat separate note, when you think about it, the idea that Dunmer regularly invaded Black Marsh to acquire slaves doesn't make sense at all due to the logistical difficulties involved with transporting living cargo across harsh to untraversable terrain in an enemy nation. The only way they could keep a steady supply of slaves coming in would be if the Argonians were actively giving or selling them to the Dark Elves. But that would go against a certain narrative and misconception concerning slavery in the real world that's been pushed for the past century.