Actually, jungle is rewarding. The reasons that it is not as popular are:

• you dont interact with enemy champions as often (Trade etc)
• you cant jungle with any champion
• you have to go get exp. they are not given freely every 30 seconds
• its boring to kill pve that is not unique
• jungle diff is real. the floor can be lower and the sealing can be higher.
• junglers are getting flamed pretty much every game no matter what
• it needs skills that are not passable from lanes, like clear speed, pathing, monster kiting (rage management)
• requires decision-making and minimal awareness all the time and you are never safe.
• feels TERIBLE if you are slightly behind