Why can't the tenant farmer fight back at those who cause his problems?

Basically because the tenant farmer has no power. The banks hold all the cards here. The farmers don't even own their land to begin with. Steinbeck describes them as simple but honest folk. Most of the farmers can't even comprehend the power structure that enables the big cats to mow down their shacks. The banks own the titles to this land and are reclaiming it from tenants who have, because of draught and the depression, ceased paying their rent. The banks plan on the beginnings of much larger farms (agribusiness) owned by large companies. The banks have the police and the law supporting them; the farmers have nothing.