They aren't. It's a preference. Their mid-fi offerings (HD555 through HD650) share a specific "house sound", which is smooth and musical. These headphones tend to be pretty neutral, if a touch on the warm side.
Lots of people love Grados too, and Beyerdynamic. No one company is necessarily technically superior to the others. Sennheiser and Grado specifically have big followings because they have unique house sounds.
They're comfortable and easy to listen to, they have a very laid back signature. They're not better than other brands. They're simply well known, popular, and well advertized.
I frankly like BeyerDyanmic, Shure, & Ultrasone better. I don't like laid back so much. And I have had my HD580's for 10 years. Least used headphone I have at this point--I use them as reference only these days.