Covering your hair maybe makes sense if there was a lice problem, but I can't think of a decent reason to prohibit wearing of mixed fibers.
Not wearing mixed fibers was never a practical rule, but part of a common theme in Judaism of separating things, like separating milk and meat. Most importantly, the sacred is separated from the mundane, and the Sabbath is separated from work days.
There are quite a few rules in Leviticus that are impressively practical for an ancient people with a primitive understanding of biology: there are restrictions on a touching corpses or people with skin diseases, and frequent handwashing is mandated, including before eating, after urinating or defecating, and after visiting a graveyard.
There is no rule about mixed fibers. Instead there is a rule about combining wool + linen, and the reason is simple enough: It was reserved for ceremonial use, and not permitted for mundane use.
Specifically the priestly garments were made of it, and Tzitzit (which are worn by all Jewish men, not just priests) are made of it.
Milk and meat are separated because it's considered "rude" (wish I had a better word, maybe "boorish") to put the meat of a dead animal in the same liquid that it makes to feed its young. It's also not permitted to kill a parent animal and its child in the same day, for the same reason.
It's like the great seal of the US - permitted only for official use.