Legalism is rather like the teaching in schools in England today - you have a set of rules (the curriculum) and if you learn all the rules you pass the test at the end. Teaching is now about how to pass the test not about how to think.
It is, therefore, easier to follow a list of rules: Christians don't --- smoke, drink alcohol, swear, commit adultery, etc; Christians do --- give money to charity, go to church every Sunday, remain faithful to their partner, etc.
The rules were developed, as far as I can understand, by men and women of faith to provide pointers not as absolutes to be obeyed without question. We are believers adn need to follow Christ's teaching not a set of man-made rules.