Reb Mendel would often say: In Russia, in the difficult years, although the chassidim were ready to go on mesirus nefesh, still, sometimes they had to pick and choose what inyanim to have mesirus nefesh for. For davening with a minyan they didn't have mesirus nefesh; it wasn't worth the risk that every illegal gathering entailed. However, on shabbos mevorchim, since they anyways had to gather together for the farbrengen, which was something that was worth any risk, they already used the opportunity to daven with a minyan.