Legalism is self-centred concern for keeping the rules so as not to make yourself impure, even if at the expense of others. Legalism is the opposite of Love. It promotes both spiritual laziness and fear. Spiritual laziness, because it makes you content with mere ritual. You can mask your lack of love with religious acts - going to church, saying the liturgy, taking communion, putting money in the plate.
Legalism also promotes fear.
But legalism causes fear. It can be very intimidating. Most of the disciples were men with professions and lifestyles that the Pharisees considered far beneath them. And the disciples, like most Jews, looked up to the Pharisees as important and respected pillars of society. To offend the Pharisees was like offending an archbishop, or a supreme court judge, or worse still, the mass media.