The Book of Leviticus was not written for the layman (in this case, the average Israelite). It was written for Levite priests. That bit of context is worth noting.
As for why people abide by some of the terms in the Bible and not others: Because it doesn't suit them, along with most people today who claim to be religious actually being "fakers" for lack of a better word. A great number of them likely haven't opened the Bible, much less actually read and analyzed the writing and put it into historical and social context. Not saying this is the majority or even a substantial minority, but I don't think it's much of a debate that religion has basically become a social requirement in the US at least, and thus many people who don't hardly know a thing about Christianity will avidly self-proclaim themselves Christians. People cherry-pick every part of the Bible, not just Leviticus or the OT.
Whether this is good or bad is up to one's opinion. From one perspective, the Bible has to be cherry-picked in order to make sense. It was passed down orally for centuries before being written down, then it was translated into dozens and dozens of languages and dialects (sometimes by people who didn't know the language being translated that well), edited multiple times, had entire books and sections cut out, etc. If you don't cherry-pick it, you're going to get contradictions constantly.