Because there’s relatively little need for diversity. Dumping toxic waste into river systems is as bad in Oregon as it is in Florida. Things that genuinely need local management (e.g. wildlife) can be managed locally.
This is where most of the dysfunction comes from, not the classification of what is or isn't a pollutant. Because this is how you get some water rich municipality in the southeast having to adopt stupid rules they don't even want based on some guidelines that were written to be compatible with the water poor west.
There'd be a ton less people who are categorically opposed to the federal agencies if not for this sort of thing. And it goes for most agencies, not just the EPA.