I would think that the reason is cost ... reproducing photo quality logos for marketing materials (business cards, brochures, shirts, etc) is many many times more expensive than a graphic with 1-4 colors.

In the end, it's your logo ... you can do ANYTHING you want which is the cool part of owning your own business ... but trust be, the anything you want part doesn't always line up with the what's financially practical for a small business as I have learned many many time.

Hope this helps.

-adam