He was not leading man material and was not prolific in film work.
Its that man again!!

He should have won for
The Heiress
.
Wonderful performance.

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I don't think so. I think it was because of the nature of the roles that he normally played. While Olivier did Hamlet and Romeo, Richardson did parts like Falstaff and Caliban. Important parts, but not the Romantic, dashing roles that help to make one legendarily famous. In most of his movies he was not the main star; just the opposite was the case for Olivier and Guiness. Gielgud was often a supporting player in his movies, but definitely not on the stage. It's also just the case, that more of their movies are better known than Richardson's generally speaking, thus they are remembered by more people.
Whatever the reason, however, I don't think it's fair. Richardson deserves to be right up there with them.