The/a reason is that the difference in vocal ranges is usually not a multiple of an octave. So, suppose a woman can sing A3-A5, where A3 is the A below middle C, and a man can sing E2-E4. Let's say the song has a range of 1,5 octaves, originally written from C4 to G5. Now the woman can sing it but the man can't, since when you transpose the song by an octave or two, it will contain either too high or too low notes for the man (one octave down and the G4 is too high, two octaves down and the C2 is too low). The only way is actually to write the song so it goes from A3 to E5. Then the woman can sing it as written and the man by transposing down one octave.