If I already asked a professor to serve on my dissertation committee but I now longer want him on my committee for various reasons, how can I politely tell him this through email? I just don't know how to say this in a polite and not at all prideful way.
You do not need to tell them from the first place. They will implicitly understand that you changed your mind. It is fine. But if you send them the official forms and dissertation, I think you need to follow a certain procedure. In this case, it is not polite and not recommended to change a committee member unless there are strong reasons.
Contacting committee members should be done by the supervisor not by the student in most of the places. I highly recommend you to consult your supervisor as this is part of the supervisor's job.
Just tell him you've decided to replace him but thank you for having been willing. Don't volunteer or reply with the "why". If you're on campus, I would just do it in person. Make sure you have a replacement available too. Then just notify whoever maintains records of such stuff, department secretary, I guess. Probably worth letting your advisor know too.