3D TV is a very fiddly technology. You need to sit in the right position, you need special glasses or just the right angle, and your position doesn't influence the image. 3D is also that doesn't fundamentally change a movie. It has depth now, it's a really cool thing in some cases, but it's still the same movie.
An HMD is a completely different experience. The effect is perfect without fiddling. You can look around a corner. And it gives an amazing sense of immersion, which for some games is an excellent advantage.
For instance Elite: Dangerous is far easier to play in an HMD. To access the menus and ship interface all you need to do is to look in the right direction. If a ship flies out of your field of view, you just need to follow it with your head. Sure, all that can be done through keyboard or joystick controls, but it's far easier and far more intuitive to do it the same way you would if you were sitting in a cockpit.