Can someone explain to me how come a biopsy is more reliable in diagnosing bladder cancer than a urine cytology?

Both are good: Urine cytology just catches whatever cells have broken off the tumor and ended up in the urine. These cells were usually dead to begin with, or they would not have fallen off. If they were not dead to begin with, they still die once they break off the main tumor. So the pathologist has to try to guess what type of tumor the cell came from, even though it is basically dead and rotten.

See below: Urine cytology is looking for cancer cells in the urine, which can be missed as they may not be shed into the bladder, from its lining. The amount shed may also be too small to detect. The biopsy of a suspicious area is more reliable.