Why? If you send a mono signal to both of the inputs, you can power 2 separate cabs.
True. But it would not be the same, and this option would anyway be available with a bridgeable amp.
Sometimes you might not have two cabs at your disposal, or missing the room on stage or in your garage... or in your car! So you shall be stuck whith half the power.
Depending on the version, 180 W mono could be enough for certain applications, but 90 W could be not.
Basically talking of a higher versatility here, and not criticizing the design as a whole. Furthermore, the option could not be considered missing by many