Why is cat urine more difficult to remove?

Cats have a more concentrated urine than dogs. Their urine is very concentrated as they originated from the desert. Cats absorb a large amount of water from the urine to maintain hydration. 

Cats have a very long loop of henle - A cat’s loop of henle is extremely efficient at squeezing every last drop of absorbable water that this concentration makes the urine smell foul. 

Dogs have a much more diluted urine which means the odour isn’t as strong and less yellow in colour than cats. 

Excerpt from www.drjustinlee.com