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.
You would think that people who supposedly espouse doing unto others as they would like others to do unto them would 'get' this, wouldn't you?