Do your research.
http://blogs.sega.com/usa/2009/02/10/sonics-ultimate-genesis-collection-shipping-this-week/
Time restraints, not laziness. Also, they said several games; which I believe gives them enough material for downloadable content. Sonic 3 and Knuckles, Blue Sphere, Toejam and Earl 1 and 2, Gunstar Heroes, and whatever else I can't quite name.
And then next year Sega needs to follow up with a Saturn Collection.