It's never made any sense to me. Aren't cows and lamb just as "filthy"? Furthermore, it takes a lot of work to domesticate and maintain an animal(s), so why are pigs around today and sold if they are so "bad"? Who maintained the species and why? Why do they have a negative stigma?
Since nobody answered yet, I'll tell you what one of my Professors said to me: The prohibition of pigs might come from a nomadic livestyle. Pigs are one of the most efficient livestock animals (for food) in a dense, urban enviroment while completely useless with a nomadic livestyle, as you can't take pigs as easily with you like sheep, goats or cattle.
I'd suggest you to re-phrase the question (asking about pigs in nomadic/non-nomadic livestyle and/or asking about the nomadic livestyle of the early Hebrew/Arab tribes) and possibly cross-post it on /r/AskAnthropology to get a more in depth answer.
It wasn't so much that they were considered "filthy" or any other medical reason connected to what was and was not kosher. Following Kashrut was another means for religious Jews (both past and present) to connect to God, because when you read the laws surrounding Kosher foods and what was traditionally allowed to be sacrificed to God at the Temple, you'll notice the lists are the same.