Just curious. Why is Tahm Kench now been fully seen at Top lane instead of Support like last year?

Generally speaking this is one of those odd quirks with where a champion is most effective, high utility champions will often get moved to support, since generally speaking a support is able to function with minimal gold flow. We've seen a good amount of this kind of shift for champions, Shen, Trundle, Zyra, Annie, etc getting moved down there. The secondary part of this is if they can function exceptionally well if given their own gold flow. Lulu and Tahm are two of the better examples of this, and Tahm's is in part due to Juggernaut/Marksmen itemization changes synergizing quite well with him.

To kinda add onto what artum mentioned, most teams nowadays typically try to use supports that have some form of hard engage potential (morg's q, thresh's q, alistar's headbutt/pulverise). Tahm lacks that, making him just generally harder to fit onto team comps.

The 2 guys above me got it best, he scales well with gold and teams are looking for engage on their supports and that's what tahm lacks

When Riot nerfed his MS boost for eating an ally, it really hurt his support power.

At Worlds carries were best played top because they were "broken" but since the carries got nerfed and the masteries favor dueling more in the top lane, tahm can do well.

He always functioned as a top laner even before now.