Because an electric handbrake is either on, or off. Whereas you can slowly release a manually operated handbrake so you don't go from a situation where 100% of the car is being held by the handbrake to 0% of the car instantly.
To combat this virtually all cars with an electronic handbrake have some form of hill hold assist, where the brakes remain applied for several seconds after release until you accelerate, or an auto releasing handbrake. For cars like this, operation is simple, easy and hassle free.
Some cars, however, do not - I'm looking at you, 4 cylinder BMW Z4's. These have an electronic handbrake, no auto release and no hill hold assist. Doing a hill start is quite tricky until you get used to it and even then is more faffy than it really it should be...