Transmission in a cycle rickshaw......

To help it during turning. While taking a turn the outer wheel moves at a greater speed as it covers more distance in the same time. If both wheels are attached to the axle then thy will have the same speed and it will be difficult to take a turn.

As told by Prateek, if both rare wheels are connected to power transmission directly, it will cause skidding while turning. To overcome that, one might have to use a differential. But diff will cause a transmission power loss due to introduction of more parts. Also in bycycle, it's not justifiable.