Because they are awesome.
I have a women's medium which I used on the Appalachian Trail. It worked well as a raincoat and as a layer to keep me warm.
I also have a men's large which I use in the alpine when I think I may need to wear my expedition weight down jacket in inclement weather. That way I have a jacket that can be worn over the down without compressing it.
- Exceptionally light (5.5 and 7 oz respectively)
- Waterproof* (see cons)
- No Frills
- Great fit
- Drawcord hem
- Full coverage hood
- I don't believe anything that says it's waterproof and breathable. These are no exception.
- These jackets are very waterproof, but they trap condensation next to the skin (or in my down jacket) if worn while exerting.
- They don't hold up to backpacking. The shoulders end up leaking after a few dozen wearings with a pack on. Unfortunately that means one thru-hike for me.
Overall I love these jackets. They work as advertised well enough that I can put up with them wearing out faster than I'd like. Also, OR has a good warranty program and they have always replaced gear that I've returned when I felt like it failed to last for a reasonable time (prior incarnations of these jackets included). OR steadily makes improvements to gear based on customer feedback, returns, and reviews which I appreciate.
If you're looking for a light "insurance policy" rain jacket I recommend the Helium II.