WWW eventually pulled out into the lead, due largely to licensing considerations; WWW had been developed at CERN, which explicitly disclaimed any ownership over it, while Gopher had been developed at the University of Minnesota, which preferred to leave its ownership claims ambiguous. People gravitated towards WWW for the simple reason that they knew no one would sue them for using it.
As WWW's userbase grew, demand grew as well to add features to it. Mosaic added images and image maps; Netscape added JavaScript; and on and on and on. Eventually WWW grew to the ginormous, do-it-all system we know and (ahem) love today. Because Gopher had been left behind in adoption, it didn't have those pressures to extend its capabilities; it was free to remain the simple hypertext system it was in the early '90s.
I see a lot of people today point to Gopher as an example of what the Web should be. But this misses the point; the Web isn't what it is because of some design decision it made that Gopher did not, the Web is what it is because it had users. The more users there were, the more things people wanted to do with it; and the more things people wanted to do with it, the more features got tacked on.
If Gopher had been the one with the permissive licensing back in the '90s, it's very possible that it would have been the hypertext system everybody used, and today we'd be complaining about how complex Gopher has become and asking "hey, whatever happened to that toy Tim Berners-Lee was hacking on back in the day? Remember how simple that was?"