They accepted you into a specific program. Once you decided to change majors they would have re-checked your qualifications for the new program and the number of spaces available. Either you did not qualify or there was no room.
By the time they went through all that, the original program was probably full and they could no longer fit you in.
For the rest of you aspiring geniuses who are assuming this is the universities fault - never change your major without first checking that you qualify AND there is space. If you skip that step then you are an idiot and don't deserve to go at all.