In my opinion, bottom lane is the hardest lane to gank. Top lane is pretty easy, you have a few distinct routes you can gank from that will get you in excellent ganking position, especially if you're playing a jungle who can move over walls. Middle lane is, well, middle difficulty -- unless you're Rek'Sai, there's 2 optimal entrances and 4 side entrances, if you're Rek'Sai there's 6 optimal entrances. This sounds like a lot, but if they ward the river it's harder to execute and-- well, I could go on for days about this.
Bottom lane is EXTREMELY open and incredibly easy to ward. Tribush ganks are only good if you're on one of the two teams, and the river is too open and easily wardable to be a good ganking route. There's a ton of space between the ganking routes and the lane, and often the enemy laner will be able to retreat/be less affected by the gank because of the travel time and length of the lane. Even if you successfully gank, it takes FOREVER to gank bottom lane and then your mid/top will be like "ermahgerd jerngler gank u useless stupidface" and then feed and it takes forever to get from bottom lane -> top lane and, and, and.
Many junglers do not prioritize bottom lane because it's just so annoying to gank, why gank bot when top is easy and rewarding?
Also, bottom lane does tons of damage rapidly. If they're losing, by the time you get around to ganking there they'll be fully pushed up to their tower and/or die and then flame you.
Speaking of flaming, adcs can be extremely toxic and honestly? I don't want to gank for a toxic feeding vayne. You're op lategame anyways mr vayne main, if you're losing I can gank mid and top and make our lategame better and get you carried. If mid can burst down the enemy adc before they can do anything in a fight because I got them fed or top is like "damage? what's damage?" I honestly don't give a damn how fed the enemy adc is.