The vast majority of his usefulness is pulling people in (long with high base damage CC) coupled with the fact that he requires low amounts of gold to function.
Bottom lane needs someone that requires low amounts of gold to function, as well as you get 2 targets to pull in. Making him an easy to pick fit for the "support role"..
Putting him top means now he put into a situation were he just duels other people with jungles roaming by to gank. So he needs to be picked against a top laner he can beat as well as a jungler that can't gank him. Then since that certain build requires alot of gold he needs to be played against someone he can farm against. He doesn't have any good minion clearing ability outside of his ultimate.
All in all blitzcrank top works best as a surprise niche counter-pick verses certain things. Which why he isn't "popular" to do nor something you would think of commonly. Personally I think blitzcrank mid verses someone like a vegiar (high farming low mobile champion with weak early game). Since they can't push you in and your pulls are deadly.