Because the increased pressure on your distal urethra (caused by your tumescence) occludes it, thereby requiring you to either overcome that extra pressure (abdominal straining) or wait for the source of the excess pressure to abate.
One thing I like about living in a dorm in college or being in the military was living in a place that had urinals. That way pissing with an errection was easy. I say try kicking your leg up like a dog.