Primarily for reasons of legibility, in my opinion.
Technically, any piece in duple meter could be written in any duple meter and any triple meter piece could be written in triple meter. In fact, you could even write a triple meter piece in duple meter if you are willing to notate everything as a triplet.
Also, as some of the answers below show, part of legibility is in supporting the speed or the feel of the piece.
(Also 2/2 is a fairly common time signature better known as cut-time. Many marches, for example, use this.)
Technically, any piece in duple meter could be written in any duple meter and any triple meter piece could be written in triple meter. In fact, you could even write a triple meter piece in duple meter if you are willing to notate everything as a triplet.
Also, as some of the answers below show, part of legibility is in supporting the speed or the feel of the piece.
(Also 2/2 is a fairly common time signature better known as cut-time. Many marches, for example, use this.)