Because it takes more than a few months to port a game? Miyamoto said it takes about 1 year.
A year? OK, a waulity port with extended features, improved art assets and new controllers.
I work as a programmer, not games though. But the main difference is, that games include a lot of art (which does not need to be remade for a port) and gets a lot more testing than normal software. So if it takes a year: 9 months the bosses write papers and have meetings to finally decide they do it, 1 month for programming, 2 months for testing. I actually know this from my work, if I hear the first time about a project it would've plenty of time, if we get the go the deadline is nearly upon us.
New games are different though. Making concepts and finalizing your goals alone takes probably around a year for a big game (although this part is low on manpower). This is a creative process.