The balrog is what kept out the dwarves, however it may be that not all dwarves knew exactly what the evil was in Moria, and thus they may not have known the extent. In FOTR, not everyone knows what the Balrog is exactly (in fact, only Legolas seems to know, he is the only one that names him). Perhapes Balin did not know just how dangerous Moria was, and that is why he went back.
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