I'd say most of it has to do with it simply modifying an auto-attack into a sudo-critical strike like thing. Just from the game-mechanics side, it makes sense to be physical in nature.
Most of the tanks and bruisers that pick it up tend to be in that fuzzy spot were you aren't sure if they should be classified as AD or AP anyways. Like Alistar, Blitzcrank and Malphite. I'm never sure what damage attributes to stack on them - they can really go either way. I feel a little bad for Shen's inability to put the mana to use, as he's really one of those tanks - and Iceborn Gauntlet offers a lot of mana.
Originally Posted by ROOOOOAST
This item isnt made for tank.... Its for all champ who could use sheen well, but not the zeal/phage (more often the zeal). They wont change it, because 41% of the time this item is on ezreal, or like 5-10% on nid, and both are not tank
(Stat taken from lolking.net
I'm like, 95% sure it was called the "tank sheen" in development. And I'm fairly sure it wasn't designed with the likes of Alistar and Malphite in mind much more-so than Ezreal. It's not like Riot can tell which champions end up picking it up all the time before it's released either.