It's not denying him his theme, he still has a theme, it's called 'Darth Vader's theme', and I think it works rather well. The mechanical sound of it shows how little humanity is left in him; whereas the IM is closely connected to 'Anakin's theme' and comes back in ESB when he has learned of his son.
The Imperial March and Darth Vader's Theme are one and the same. Anakin's theme was played mainly in TPM
the little piece that sounds like a modified version of the nutcracker suite.
Obviously when Williams scored ESB, he found that Darth Vader had no clear iconic tune that people would be remember.
When watching the movies in order, it can be seen that its absence is noticeable.
But that's the essence of my point, it sounds different because it represents how he is more man than machine, then in ESB where he learns of Luke, his humanity begins to come back, and is accompanied by the IM. So to me, even if it wasn't intended from the begining to be that way, it makes sense that the Imperial March isn't in ANH. I guess we disagree though, huh?