By and large the current design has one set of drive wheels and one steering set. More of an Ackerman model, even though kinematic models aren’t directly used in the project per say like they might be in a ROS project. The default car receive its velocity commands for essentially a fixed set of rear wheels.
Whereas differential drive is more suited for robots with two primary wheels that provide velocity and angle from a single set of wheels and have a third caster or similar type wheel for balance.
I suspect the real reason is that most of the model cars more closely resemble the ackerman, but as indicated at the top, even that is controlled through simple “+1/-1” for forward/reverse and “+1/-1” for left right without any linear algebra one might normally use to convert angle/velocity into separate motor commands on a differential drive robot.
That said, there’s nothing stopping you from building your own diff-drive robot and using the part. (I’m in process on such a car myself, mostly because it’s much easier to get odometery)