The Single edge shaves closer but it requires bit more technique that most prefer not to do and since the double edge may be easier to get and learn that's the popular choice.
I think this is another contributing factor too, lack of choice on bladesI think SE razors are the ones that are easier to use, I have an MMOC and to me it's like a duplicate of a Mach 3 cart razor as far as on the face feel goes. I use SE razors when I want a quick and mindless shave. So as the adage goes, YMMV.
To me the DE Shavette is the ultimate in closeness but also can be the ultimate in testing technique. The DE in aggressive razors ranks second and the mild DE's and most SE razors rank third in need for perfect technique.
Then when you consider the lack of blade choice in SE razors they just don't appeal to my style of shaving. But I do understand that others do not agree with my ranking and that is OK with me also.
Too me if you like them, then by all means use and enjoy them, they can and will give excellent results, just not the variety that I get from my DE razors.