Why? Many sects of Christiantity have very odd beliefs. Look at the Mormons and the Christian Scientists (not that transubstantiation isn't an odd belief, either).
Because most conservative Christians only use the word to refer to sects that are fairly orthodox, i.e. who at least subscribe to the Nicene Creed.
If you don't include caveats, they'll interpret you to mean that Hitler was a Christian in the same sense they are. No one wants to be put in the same group as Hitler; it will turn them off to your argument, especially if they realize that Hitler's religion is fundamentally different from what they consider Christianity.
In addition, the argument that Hitler perverted Darwinian theory just as he perverted Christianity makes for good rhetoric.
One man's theology is another man's belly laugh.Robert A. Heinlein, Time Enough for Love, 1973