Because junglers arent responsible for a single lane, they are held responsible for 4 of them ( counting jungle as a solo lane). And if anything goes wrong in any lane , its because you weren't there to help , right at the moment they needed you.
Mid has to watch above and below, top below, and bot above. But you have to watch everything, all the time , and be right where you are needed at any given moment .
The backlash you will get from ganking top when bot is getting hit by mid and the other jungler ( despite your mid just staying in lane and farming, without making any progress on the turret , or even bothering to ping mia ) is too much for a lot of folk .
Hell, you cant even stop to get the red buff that will aid in your gank , because the second you are on that side of the map, your team will see it , and through no input of your own, decide that you must be coming to gank, and try (and fail) to cheese the enemy. Then its a storm of pings and shittalk .
Its just not worth it to a lot of folk.
Another side is the rune changes . Without thier old setups, a fair amount of people arent sure what to take to jungle anymore , and just decide against it.