Fidelity and ergonomics. In addition to a DSLR I also use a Fujifilm X-E1 with an electronic viewfinder. It's much preferable (IMO) to hold a viewfinder near my eye, blocking out external light, with my elbows stabilized against my body than to hold an LCD panel out in space squinting to see the image in bright sun.

In addition, although I use the X-E1 a lot for travel, the DSLR's optical viewfinder is still superior optically and the camera as a whole is faster, more responsive, etc.--in part because it's bigger and the designers have more space to work with.

I do think we're headed toward a point where electronic viewfinders will tend to replace optical ones but I doubt we'll replace looking through viewfinders.