Answer: because one drum has a lot of advantages.

The only beeing disadvantage is that the rearwheel does not centere it for themselves. The advantage is that the weight stands directly on the drum.

On two drums, the weight force is divided into two clampforces which causes an increase of the roll drag (upper picture).

The smaller the tire, the worse the effect  (upper picture).

Under load, the motorcycle starts getting shorter by deflection of the frame, fork, front tire, etc. and the result is, that the rear wheel is pushing themselfes on the front drum (upper picture). To get allways the same conduitions, it is much more easier to work only with one drum (picture below).

This is the reason why we can do very precise measurements. On two test runs with a 50 ccm scooter, you can see the differences between a full and a flat battery. The test run with a flat battery shows 80 watts lower performance. This is the energy, the battery needs for charging.