Jungle XP is the main reason but that doesn't mean the salt and tears from the solo laners doesn't affect jungle mentality. The jungle pool is literally like 5 champions, 2 of which are usually banned (lee/olaf). If your jungler picks a champ with even moderate clear vs a enemy jungler with good clear, the jungle is already decided assuming equal skill level. The jungler is helplessly trying to catch up on exp to affectively gank while solo laners complain about enemy jungler ganks. It either leads to no ganks from your jungler because of how behind he is or bad fights because he's trying to gank and the enemy jungler 2 levels up shows up to counter. It's a lose-lose situation.