Why use an airplane with non-retractable landing gear for aerobatics?
As you mentioned, drag is one of the reasons why retractable landing gears are used in the first place. But in order to use it, there are way more considerations than just drag.


Scale: Size of the aircraft plays a big role here. Big aircraft have more room to keep the retracted landing gear. This is an issue as planes get smaller.
Weight: Retractable landing gears not only require more space, but they impose more weight
Cost: They add extra costs in installation and maintenance.
Application: High speed applications justify a retractable landing gear for a more aerodynamic shape. Does the present situation really need the application of a retractable landing gear?


These are some considerations that go into it, but not all of it. In the case of a high performance sports plane (like in the video above):


Scale: the planes are small. The retractable landing gear systems (system) need a lot of space. This is not what we want in a sports plane
Weight: A high percentage of the overall weight is taken by the system. This in turn increases the inertial forces* and decreases the overall maneuverability.
Cost: It is expensive to install. And it may cost even more due to the low volume requirements.
Application: More than high speed, high maneuverability is needed.




* Inertia is the tendency of a body to stay in the existing state of rest or motion. So inertial force is the force that keeps the body in its existing state of rest or motion.

[EDIT]
Based on a very good response below (from @Devil07), I am adding this important point here (just for a better overview of all important aspects):


Complexity: Retractable landing gears are complex piston mechanisms. Eventually it adds to the cost of installation and maintenance of the system.