1. makes it easier to go after organized crime or large drug enterprises. The feds need stuff like warrants and witnesses for normal crime things. But tax evasion is an easy way to start an investigation.

2. People don't report their illegal income as "crime money" on their tax returns. They'll write it down as normal business expenses.

Now this is just hypothetical law stuff, but the IRS is under no obligation apparently to report criminal money to any other federal agency. The reason for that is the 5th amendment, where people have a constitutional right against self-incrimination. Since reporting income is mandatory, you're obligated to say where you go the money. But you have a constitutional right to not be forced into incriminating yourself, so hypothetically you're not supposed to get investigated/arrested on the sole basis of self-reported illegal things on your tax return. Since that would be evidence based on a forced self-incrimination.