I want to say this is a limitation with the motor design but I could be totally wrong. Everything I'm reading says this shouldn't be the case though, so maybe it's just a software thing.
Typically the maximum torque output of an induction or permanent magnet motor is constant all the way down to zero rpm.
I've heard suggestions that Tesla might have some fancy new variation on the permanent magnet motor here, so possibly it's not true for that, but I haven't seen any evidence of that yet.
Looking at the dyno chart drag times gave us: Tesla Model 3 Long Range Dyno Testing, it seems likely they used software to limit torque below about 20 mph for whatever reason, unless there was a problem with the test.