Beyond strategic reasons mentioned elsewhere, the main ones are (a) local candidate of desired party is unqualified/unpopular/etc and (b) local candidate of less desired party is more qualified/more popular/etc. (Seeing as "popularity" is, for better or worse, a pretty important quality in a political candidate; this includes "I went to the same high school as candidate X, they seem like a good person.")

This happens easily in more-than-two party systems like Canada or the U.K. Since each party overlaps with another a bit on certain issues, and many voters only seem to feel strongly about a few issues, those voters are ideologically okay with more than one party. Even if they have a "favourite" party, they will consider the candidates of those other parties as well.