really, it's a NYC actor thing. And for their guest leads and supporting, they look for really strong actors with "names". It's the same, relatively small pool. They usually have to cast like 20 parts an ep for the Law & Orders, so folks are going to be recycled after a while.
It's always fun to see old eps of L&O and you see an actor who was a working actor then but has a much bigger profile now. Like back then you weren't like "oh! that's ellen pompeo!" cos grey's anatomy wasnt even a thought yet or Shane from The L Word, I love the two episodes she did back in the day.