Superbit 2xdvd
 

If Superbit cant hold any extras due to the extra quality, why dont 2 disc sets such as spiderman have the main feature on a Superbit disc and the bonus material on a regular dvd??

Some do, some don't.
For instance, Snatch and Mask of Zorro both have a second disc included (making them what Sony calls a Superbit Deluxe ), with all the special features preserved from the regular 2-Disc SE versions, less director's commentary (which of course would be have to be on Disc 1 anyway).

Strangely, Sony hasn't produced a Superbit Deluxe title in a while.
Makes me wonder if they're trying to get consumers to double dip.
Just a thought... ;)

So Sony can potentially make two sales instead of one. Besides, a Superbit usually drops commentary tracks, so it makes sense to have a dedicated SE and a dedicated presentation disc. Sometimes it doesn't work out that way, however.