It is interesting to note how many actors are best known for their Batman characters

[Dan E Kool]

I'd imagine that IMDB compiles those lists based on what their users are searching for. Seems as good a measuring stick as any.

In regards to Cesar Romero, though he had a very successful career outside of Batman, his Joker getup is so iconic and instantly recognizable, it's not surprising that it would be his most memorable role. The same can be said of the other villains. These are their best known roles for the same reason that Adam West was typecast after the show - the costume and character is tough to forget.