Because it does a lot of things very very well, it doesn't take a huge skill set to get really nice images out of, it is small, light, good value and multi functional.
It doesn't rely on super fast aperture, so it isn't a single use lens, that is, the only difference between a 35mm f1.4 L and a 35mm f2 IS is size and weight, one stop, lack of IS, and $800. It has a unique IS. It makes a superb portrait lens. It makes a really fun and easy to use macro lens with a decent working distance. It is a small and light moderate speed short tele. It fits in a bag easily and weighs next to nothing, compare it to a 135 f2 or a 180 Macro.
What is not to love?