Why can the Catholic Church declare one person a saint but has never declared another damned? The Catholic Church has a process to canonize someone, to declare that they are a saint, that they are 


  truly in heaven and worthy of public veneration and imitation.


(Cf.  Canonization - United States Catholic Catechism for Adults).

If the Church can declare that someone is "truly in heaven", why do they not also have authority to declare that someone else is likewise damned and truly in hell?
That is because Jesus gave Peter the keys to heaven and earth (Matthew 16:19). Not keys to hell. In other words, Peter (and his successors) can bind and loose on earth and heaven (Matthew 18:18) not in hell. They do not have authority on beings outside of the church. And people in hell are outside of the Church.

P.S: Christ alone has the keys to hell (Revelation 1:18)

PPS: One can never be certain that another is for ever damned since there is always a chance for someone to be converted, even at the last moment of their lives. This is a matter between God and the dying person.