Because any electric hydraulic pump is going to be slow at pumping fluid, you still need the pressurized accumulator, so you can move your control surfaces quickly when you need to. So the reservoir, pump and the batteries are an additional weight, that you would want to omit if you could.
The reason why you have never seen a total loss pressurized system is that the conditions that call for it are ones that you rarely see - a strong mass constraint (which has to include the power source), and a short time period when it is required. Your plane's system needs to operate over a period of many hours, there is normally a power source on hand (the engine's alternator), and the mass is not really that constrained. Really, a rocket is the only place where a pressurized total loss hydraulic system makes sense.
(Note that this crash could probably have been avoided with more complex programming. The programming could have kept count of how much hydraulic fluid it was using, and driven the fins to neutral before it ran out. This sort of capability - or even just a fluid level sensor - will doubtless be added before the landing system leaves the testing stage. If dealing with early exhaustion of fluid does turn out to be this easy, then the need for the extra mass completely disappears.)