Why is sheet music for pieces written by composers who are now?
In most countries, copyright on someone's work extends for many years after their death; here in the UK, that term is 70 years beyond death. That means (as of the time of writing in 2015) that if the composer died after 1945, their work is still in copyright. In addition, copyright on the printing and typsetting of any new edition extends for 25 years after first publication. This is what Alon refers to in his answer. Any edition published after 1990 is still in copyright, regardless of the date of death of the composer.