L298N H-Bridge : Powering 4 motors using (6) aa batteries vs (4) aa
The L298 is a very old technology part.  It drops about 3V total from battery to load -- so a 9V source would only be 6V at the motor in the end.
Dry cells only deliver 1.5V/cell when they are freshly charged and moderately loaded.  At heavy load, or any load when they're discharged, they supply less.  If you want to suck the life out and not throw away batteries with charge left, design for 0.9V/cell
6 cells times 0.9V/cell works out to 5.4V.  Subtract out the 3V drop for the L298, and you have 2.4V.