Good question! It's because phonologically they begin with a consonant, not a vowel. These words are pronounced as if they had a 'y' sound at the beginning (Yuniversity, yuniform etc). You don't say 'an year', so you don't say 'an (y) university;' either - you say 'a university'. Many people don't realise that the a/an distinction is to do with pronunciation, not spelling.That's why we say 'an hour' and 'an honour' - these are words which are written with an initial silent consonsont, so they are pronounced with an initial vowel sound.

If you look at their actual IPA spelling, it's fairly easy to understand why. university:ˌ/juːnɪˈvɜːsɪti/ uniform: /ˈjuːnɪfɔːm/ universe: /ˈjuːnɪvɜːs/ They all start with a /j/ sound, which is not a vowel and hence, the appropriate article to be used is "a", not "an".