There's no need to change the lightbulb, we're perfectly happy with the lightbulb, although there was a problem with lightbulbs on previous crafts, we know it will definitely not present a problem this time round, because we've spent billions on fixing this problem. Oops. There goes a lightbulb.
To offer a less inane answer to your sensible question, instead of more of the nonsense above, its a case of pilot, first officer, flight engineer and 4 cabin crew to serve the snacks and attractive inflight gift selection. We have a broad range of fragrances on board from Yves St Laurant, Chanel and Jean-Paul Gaultier, all available at duty free prices. We accept payment in sterling and euro,as well as all mainstream credit cards etc, etc...
Cause it's a lucky number? Or the shuttle cannot fit more than seven? Or that's how far they can stretch the budget to get all of them life insurances that cover space accidents.. you can tell I'm just guessing.