Have you considered the case of Noah who lived quite some time before Moses wrote about him?
He was told to take two of the "unclean" animals but seven of the "clean" animals on board the ark. Since humans did not eat meat prior to the flood, the designation of clean and unclean must have pertained to sacrifice. Unclean animals could not be offered to God in sacrifice, hence the need for only two of them.
Having seven of the clean animals meant six for breeding and one to offer to God. Six for breeding would also add more quickly to their numbers as humans began to eat flesh and offer these animals in sacrifice.
I know pig meat was subject paracitic infection that could be passed on to humans, so this could have had something to do with avoiding it.
Trichinosis - Wikipedia, the free encyclopedia
Just my 2 cents worth.