If you're bottom side and your opponent starts blue, you basically have carte blanche of their jungle and scuttle until they get their red, because virtually no level 2/3 champion with red can lose a fight against an opponent without it.
Starting red is too powerful, and it's not viable for many champions to start it on the top side, because they receive a poor leash and take too much damage in the process of killing it. This gives bottom side jungle a pronounced advantage, unless the top side's jungle pick is someone who can fully heal from chickens (i.e. Kayn) or not take damage (i.e. Graves).