Universities don’t have to give you a reason, but if the rejection has left you wondering you could contact the university to ask for feedback. Try not to take it too personally. Admissions tutors will be sifting through hundreds of applications and it might not always be obvious why it’s not good news for you. If a university doesn’t offer you a place:
Accept another university offer
Apply to a new course
Apply to the next intake (spring / fall)
Take a gap year and reapply
Consider alternative routes to a degree
Consider applying to a different country