It’s a good question.
The soft libertarian crypto-advocate answer might be: Because this discrimination based on ‘badness’ of transaction purposes is antithetical to the whole original ethos of Bitcoin.
The practical answer, in my opinion, might be: Because this powers the adoption of Zero Knowledge obfuscation technologies that make tracing transactions almost impossible. So then the only solution is a total ban on all non-approved, untracked financial transactions, which is a frightening intrusion into personal liberties.
And, obviously, many people’s answer would be: Yes, that sounds good, and it will protect us from many bad things.