Excerpted from here:
What Lithuanian yeshivas do or say makes a difference because they are the gold standard by which everything else is measured. Why do some groups learn? They have to show “we learn Torah too!” Even Reform and Conservative have to measure themselves by means of the standard set by the Litvaks (Lithuanian Jews). If the Litvaks think something is kosher, that makes it kosher by definition. If they think it is not it makes it not kosher – by definition. Everything has to get past that hurdle. Nothing and no one is exempt, even if they don’t like it. Especially if they don’t like it.
You want to claim something is Jewish? You say the Chazon Ish said it was OK! Or Reb Moshe. Or Reb Aaron Kotler. You don’t say “They learn it in a Breslov yeshiva.” You don’t say some Reform Rabbi says it is OK.
For example when the Na Nach Breslov Chasidim want to show that the “petek” (letter that Reb Odesser thought he received from Reb Nachman) is kosher they go around plastering up copies of the letter of approval (Haskama) that Reb Moshe Feinstein gave to Reb Odesser. Just walk into the synagogue of Reb Nachman in Uman and you will find copies of Reb Moshe’s letter plastered all over the place–and most importantly right in the official notices section. That means if you want to say something is kosher you have to get approval of a Litvak Gadol. Without that nothing can even start.
No one says Reb Moshe is kosher because Breslov learns his books. If you want to say a person in Breslov like Rav Cheshin knows how to learn you say he learns at the Mir yeshiva. You don’t say someone knows how to learn because they learn in Breslov.