Maybe because they won't know for sure if the contamination comes from the derailment. East Palestine was an industrial town back in the day, so there's probably all sorts of nastiness still around.

Because there’s no baseline to measure against. If the air/soil/water were consistently tested it would be a different story.

From another article:

Further context reveals that the EPA previously claimed they wouldn't test for dioxins because they hadn't tested for baseline levels in East Palestine prior to the crash, making the cause of current levels uncertain; a rationale that at least one scientist refers to as "lame."

"Our toxicologists are taking a look. Unfortunately, we don’t have any baseline information about the levels of dioxins which are produced also by wildfires, by backyard grilling, by a host of other things,” Shore said. Lester says he has no knowledge of backyard grilling producing a dangerous amount of dioxin. “I’ve never heard anybody, any researcher talk about cookouts. Because that’s an infinitesimal concentration, if at all. Because dioxins form not just cause there’s burning, you need a chlorine source,” Lester said.

Lester says even if there is no baseline from prior testing to compare levels, the EPA should still be able to do testing to determine if the level that’s there is a risk."

Source

This whole incident casts a lot doubt on the integrity of the EPA.

Probably because he believes there were already dioxins in their soil to begin with. So there's no way to know how much more was introduced due to the derailment.