Jothar!
Because we live in an open market world. First we decide which product we want, then we shop around the market to find who sells it, and for the best price.
Some WANT Yiddish to be holy, due to variety of nostalgic factors. Next come finding proofs, or those who may agree, even if they do so for totally different reasons. Those who disagree are ignored outright. Lastly, we negotiate the price, since most of those who would be in favor of Yiddish only frum world would also like to see their questioneers dress in a certain levush, and commit themselves to a definite lifestyle. Since such a price is a little too high, those sources aren’t quoted at all, and the search goes on, for those who don’t demand more than the language.