sometimes diagnostic tests are better developed for urine than for blood (urine is easy to take, therefore its smarter for researchers to develop tests to do on urine rather than blood)
blood is more complicated - theres more stuff in there that conflicts with your test/reagents etc
drugs seeping out of storage sites in fat may be very quickly moved by the kidneys into urine - meaning that they stay in blood for a very short time, meaning that there is only a very small amount in the blood at any given time, while it builds up, and builds up, and builds up in the urine (you start again when you pee, obviously)
so yah. makes perfect sense.