Why did Aristotle say being good is a habit you practise, not a rule you follow?

Because Aristotle thought goodness is a kind of character, and character is built by practice. Virtue ethics is a theory that puts the development of moral character and dispositions, in other words good habits, ahead of systems that stress individual actions, duties, or obedience to divine authority. The question is less "what should I do?" than "what kind of person am I becoming?"

Aristotle's recipe, in the Nicomachean Ethics, runs in steps. First you practise righteous actions, perhaps under the guidance of teachers, to build good habits. Practised habits form a stable character, in which those habits become voluntary, and that achieves eudaimonia, his word for the highest human good, often translated as flourishing or happiness. A bit like learning a language, you do not become fluent by reading the grammar once.

He also thought each moral virtue sits at a mean between two vices. Someone who flees is a coward, someone who fears nothing is rash, and courage lies between them. Finding that mean in a real situation is the job of phronesis, practical wisdom: an acquired trait that lets you identify the best thing to do in any given situation.

This is a very different answer from the trolley problem's two camps. A consequentialist asks which outcome is best, and a deontologist asks which duty binds you. A virtue ethicist asks what a generous, brave or honest person would do, and notes that such a person acts with pleasure and without inner conflict.