Becaue he looked like this:
I don't have any problem with criticizing your own party when they do things wrong - but attending the other party's convention to air your dirty laundry accomplishes nothing but weakening your own party. If Zell Miller really wanted to accomplish reform and change in the Democratic party rather than just do some political grandstanding he could have delivered that same speech at the Democratic convention.
I agree with his assessment that the Democrats have gotten away from their core values - just like I agree that the GOP is nothing like the party of Abe Lincoln. Some of this is good and some of this is bad - but if I have a problem with my party, I work to improve my party, not improve the party that I supposedly don't agree with at all. And if I have a big enough problem, I switch parties, then start my outrageous ranting.