My guess: it's a mouthful and not catchy. "Linux" is short, catchy and easy to pronounce. With "GNU/Linux" I don't even know if I'm supposed to spell out the GNU or pronounce it as a word, and I don't know if I'm supposed to say the "/" as "slash" or "plus" or "and" or if it should actually just be silent. I like to type how I speak, so if I don't know how to say it I'm not going to write it, and I'm not going to like reading it.
I can totally see the merits for "GNU/Linux" but don't underestimate the importance of catchiness. Maybe if it were shortened to "Ginux" it could stand a better chance, but then we'd have another gif situation.