Another reason was detestation of idolatry: because the Gentiles, and especially the Egyptians, among whom they had grown up, offered up these forbidden animals to their idols, or employed them for the purpose of sorcery: whereas they did not eat those animals which the Jews were allowed to eat, but worshipped them as gods, or abstained, for some other motive, from eating them, as stated above.
There are a few prohibitions that I was unaware of until I read Aquinas. Deuteronomy 22:6 says, "If you come across a bird's nest beside the road, either in a tree or on the ground, and the mother is sitting on the young or on the eggs, do not take the mother with the young." Yet another commandment tells the Jews not to prevent an ox from eating some of the grain that he might be treading at the time, which seems reasonable to me. Aquinas makes two points: God desires His people to have compassion, and being compassionate to animals will inculcate compassion and pity towards men. And secondly, these practices were directly opposed, once again, to idolatrous practices on the parts of gentiles, which can be seen in the following excerpt:
It may, nevertheless, be also said that these prohibitions were made in hatred of idolatry. For the Egyptians held it to be wicked to allow the ox to eat of the grain while threshing the corn. Moreover certain sorcerers were wont to ensnare the mother bird with her young during incubation, and to employ them for the purpose of securing fruitfulness and good luck in bringing up children: also because it was held to be a good omen to find the mother sitting on her young.
It becomes clear to me that some of the perceived oddity of the ritual laws, results not from a strange and bizarre god, but, on the contrary, are indicative of how bizarre the practices of men had become in the world of Abraham! The very argument against a rational, sane and reasonable God gets turned on its head, and the oddities become indictments of men.
I find this fascinating.