More range means bigger batteries which means more cost and either bigger vehicles or compromising the interior space. I think MINI is smartly focusing on a niche that fits well with the brand (the SE as an EV seems similarly radical as the original Mini). I also admire how they engineered a BEV to use the existing ICE production line, minimizing the upfront development costs.
As for referring to sales volume as meager seems premature this early, especially during the pandemic. How many of us have actually gotten the cars we ordered? Seems interest has been high enough so far to keep the factory overwhelmed.
If people want a long-range EV, there are many options. MINI's focus on drivability and affordability actually provides them with a value proposition different from other EV makers.