Why do you not sell the 21Ah battery any longer?

The 48V / 21Ah battery has been replaced by the 52V / 19.2Ah. The 21Ah uses cells that are a lower discharge rating than the 19.2Ah cells.

The 21Ah battery has a slightly longer range for the pack, but has a significantly lower performance near the end of the discharge cycle in comparison to the 19.2Ah pack.

Link: Battery Range