Because it wasn't your run-of-the-mill oil spill.
It was a tar sands oil spill. Or, to be more precise, a spill of tar sands oil. Enbridge failed to mention that a year ago.
The tar sands oil thing is a problem because
1. By some reports it was the largest tar sands oil spill in the US, so it's new clean-up territory.
2. Tar sands oil is a lot thicker and more viscous than your run of the mill oil spill oil. Supposedly it's the consistency of peanut butter at room temperature.
3. It's got a higher concentration of heavy metals, and is considered one of the dirtiest oils in the galaxy.
4. Tar sands oil has extra "crap" in it to make it flow through the pipelines. (hint: it's not healthy crap)
5. Did I mention that the oil company failed to tell early clean up crews they were dealing with tar sands oil. They sort of left that part out, so the clean up crew wasn't prepared.
When that combination, known as DilBit, spilled out of the ruptured pipeline, the benzene and other chemicals in the mixture went airborne, forcing mandatory evacuations of surrounding homes (many of which were later bought by Enbridge because their owners couldn’t safely return), while the thick, heavy bitumen sank into the water column and coated the river and lake bottom, mixing with sediment and suffocating bottom-dwelling plants, animals, and micro-organisms.
It should be noted this is a river system that flows out to Lake Michigan right where people draw their drinking water, and it's just 50 miles south of where I get my water from. I guess I don't want to drink more benzene than I have to.
The ecological cost itself has been devastating, creating dead zones in the river bottom, massive fish die offs, bird die offs, musk rat and aquatic life die offs from a toxic environment.
For more complete reports on the ecological damage, follow this link.