Why did the Chassidim take upon them self not to eat Gebrochts. And not the Misnagdim? Even the Briskers or the Chazon Ishes who are Makpidim on everything, why are they not Makpidim on Gebrochts?

The general rule is “the burden of proof is upon he who veers from the norm” (Bava Metzia 6, 2), so the question is on the chassidim, not on the mitnagdim! Especially when this strict custom comes at the expense of a leniency in the Torah mitzvah (mid’oraita) of one’s simchat Yom Tov (pleasure) where in addition to the many limitations on chametz foods, gebrochts comes and unnecessarily prohibits many additional foods which are really allowed! The chassidim were apparently wary for some thick matzot weren’t totally baked inside and that some flour may become chametz when it came into contact with the liquid in the soup or recipe. In short, if you’re Chassidic and it’s your custom, consult with your personal posek and he may agree to annul your custom (through hatarat n’darim), for especially with today’s thin machine matzot, there’s even less room for this custom which not only limits simchat Yom Tov, but often causes family strife, as well.