Probably because Instagram has a third party api that allows printing and other services to access photos easily. And as pointed out instgrammers are not "pixel peepers" so a low quality projector image is not going cause complaints.
That still does not explain why they chose to offer their services only to Instagram users when accepting a pic directly uploaded to their site(s) is a fairly easy task. They suggest a work around of just creating an Instagram account. But I don't want an Instagram account. OTOH, I don't want their projector either. A magnet I could use... but I can get those lots of places that don't require an Instagram account.
Just better marketing really. Obviously there isn't any technical reason why it is