I don't see the need for something as complex and attention-hungry as a hydraulic system on a bicycle. With 'V' brakes, you fit and forget practically. The odd glance over and taking up the slack as the pads wear down.
Mechanical disks are more complex obviously but you still have a simple physical link between the lever and the part that's grabbing your wheel. But on hydraulics you've got a nasty, corrosive liquid under pressure with seals, unions and reservoirs all over the place just waiting to leak, stick come loose or let air into the system.
On a physical system the two critical parts, the lever and pads, are held at a fixed distance. I'll bet you any money that the pads on a hydraulic system need adjusted a lot more? I'll also be that it's a lot easier to damage a hydraulic system, bend a rotor, nick a cable?
I've stopped just fine with 'V' brakes for years. Hydraulic brakes just seems like a sledgehammer to crack a nut to me.