I'm not going to argue, mostly because I honestly believe that the question of why a pig is not kosher is a dead end question. All answers will be little more than theories, and rely on some shaky assumptions.
However, what is an interesting question for me, is why did the pig become THE symbol for non kosher food?
A look at the Torah, shows that there is nothing unusual in the Psukim that forbid eating pigs:
The antiquity of the Pig=Unkosher symbol is evident in the Bible. In Isaiah (64:17) we find for example:
|יז הַמִּתְקַדְּשִׁים וְהַמִּטַּהֲרִים אֶל-הַגַּנּוֹת, אַחַר אחד (אַחַת) בַּתָּוֶךְ, אֹכְלֵי בְּשַׂר הַחֲזִיר, וְהַשֶּׁקֶץ וְהָעַכְבָּר--יַחְדָּו יָסֻפוּ, נְאֻם-יְהוָה.||17 They that sanctify themselves and purify themselves to go unto the gardens, behind one in the midst, eating swine's flesh, and the detestable thing, and the mouse, shall be consumed together, saith the LORD|
In the Talmud, there is no shortage of Midrashim that show that the Pig was clearly an established symbol. So I'm guessing that somewhere between the biblical times and the Talmud, the pig was elevated to being the ultimate symbol of non kosher food. Some have speculated, that it is because the roman legion which burned down the temple (Legion X Fretensis) had a boar as one of its symbols - and hence the pig became the symbol for the destruction of the temple. (I somewhat doubt this explanation, but it does make me wonder about the Aggadah that the Romans sent a pig as a sacrifice, and when his hooves touched the walls of the Temple, Jerusalem shook).
I'm happy to hear any suggestions.