Because they don't want to, they're lazy. It is just easier to keep cutting pages. At the bottom of it, they're ridiculously lazy in how they run the site and are not actually willing to make moves to be a serious wiki, but as a business where they make small profits while counting for users to do the job of attracting people to visit the website. Everything about the site is half-done, that is why their first instinct when they see something outside their strict norms is "ban it", because aggression is the language of the caveman, who does not know how to use his brain.
How despite alternatives existing like us and AllTheTropes, they are are barely competition for it, so there is no motivation to change. Google ensure this by keeping the site on top of searches for the term trope, though that is hardly their fault; Tv Tropes is just a blimp on the radar for Google; more simply, people learn what tropes are from TV Tropes and keep spreading the site exists by linking to it, what makes Google put TvT on top of the search results for searches for the word 'trope'.
