Fairly mediocre answer. The best reason is far simpler: Because it’s wrong.
YHWH isn’t “Yahweh” any more than Kent Hrbek’s name is Homerbek. The fact that it’s impossible to pronounce is precisely the point: The letters were chosen because there is no possible combination of implied phonemes in the Hebrew language; the best you can get is “Yahuwehee,” in which case the speaker sounds like the blooming idiot he is for trying to pronounce a name given precisely because it cannot be pronounced.
Well, we can apply a similar argument to say that “Jesus” is unacceptable, since that wasn’t his Hebrew/Aramaic name. I prefer to believe that our merciful God knows what we are trying to say and doesn’t hold something as silly as a mispronunciation against us.