I'll try and simplify it.
Imagine you have two pieces of land in an area. One has never been touched and the other had a gas station on top that was there for decades and is gone now.
You, the potential buyer and developer, look at both, but one has a mystery cost below the land that you do not know, how bad the contamination and thus remediation will be until you start digging. And once you buy it and start digging gov't inspection agencies kick in to make sure things are not now leaking or moving and even if you determine 'nope, this is bad, I can't remediate this land and still build my town house complex and make a profit', ...but the gov't can still now order you to clean it up, since you are the owner and disturbed it. So now you have to clean it up even if you don't have the economics to build on it and the gov't can force you to by confiscating your assets and liquidating them and putting the money to the clean up.
You will see all over, gas station sights in the centre of growing cities, shut down and the land sits vacant for 7 years typically for just that reason. The site would be perfect for a new condo or office tower but no developer will touch it for 7 years as they have found most of the problems under the ground have dissipated by then and the cost is negligible.
Does that make sense?