Making the molds for injection molding is ridiculously expensive, and downtime on an injection molder to get them setup and running isn’t free either. You need to know you can move a LOT of a product to cover these costs at a reasonable amount per-unit.
My guess is that the cost to cast them in aluminum is more reasonble per-unit at the quantities they think they’ll be able to sell per year.
Not sure if my assumptions make sense but I presume that this charger could draw a lot of attention once it's on the market. With a target price of about $ 50 this may cause quite some new competition with regard to current high quality chargers. I wonder if using aluminum would not increase the material costs. In terms of heat dissipation while discharging this is however a nice move.
Side note: I know it's too late already for adding more features too the very large feature list. I'm not sure if terms like "factory voltage calibration" or "user defined voltage calibration" have been brought up already. The SkyRC MC3000 lets users calibrate their device for more accurate voltage readings.