Why is it pronounced but pronunciation?



That's actually a great question, Lyn Tamimi!
Firstly, 'pronounced' could be the past tense form or past participle of the verb 'pronounce'. This word could also be used as an adjective. You can say it as: /prəˈnawns/. The '-ou' in the second syllable sounds like 'aw'. This rhymes with '-ow' in 'cow, or 'now'.
On the other hand, 'pronunciation' is used as a noun. Because of 'un' in its second syllable, it is pronounced as /prə- nan- si- ey- shen/.