That's probably because scapel blades are easier to obtain. Single edge razor blades are something of a speciality item. There are also space and volume considerations.

Having said that: scalpel blades can be easily attached to a bit of whatever to make a small, usable knife.

One other option worth considering is the Derma-Shave razor. They also come as a small hacksaw blade. Slightly large than a scaple blade but much better.

http://www.donrearic.com/airman.htm

They are still made and obtainable on E-Bay.