When you're that fat, your an easy target for a gun.
But with all due seriousness, Tahm is the new Ryze/Sejuani/Ornn (pre-changes btw). Even though his influence in solo queue was healthy enough, he was incredibly strong in pro-games where communication and teamwork is actually used by players. Since the meta in the pro-scene wasn't healthy because of Tahm's influence, he was unfortunately nerfed because of it. He's been in a very weird spot ever since, as he still does find an occasional spot in pro-play for the reasons I mentioned, but still suffers in solo-queue.
Since buffing him back up to his former glory will just bring back all of the old problems again, he might need something of a mini-rework, which was how they got champions like Ryze and Ornn to weaken their pro-scene while also improving them insolo-queue.
Mind you, this is much easier said then done, and contraire to some of the egotistical players on the forums who I'm sure I'll somehow offend, there is no easy answer.