Because is big, stuck in it's own past, with many useless quirks. Additionally the malleable system has a price, which for most is bigger than it's benefits. Emacs is always stuck in a loop of fetching up with the rest of the world, living in it's own utopia and creeping away everone who can't dive deep fast enough.
Most people are pracmatic about their tools. Tools should just do their job and don't get in the way. Emacs doe not deliver that, and the people can be quite dogmatic about certain things. It changed in the last years, fresh blood and concepts are spreading their influence, but we will see in another decade whether this actually had some longlasting effect or was jsut another circle of quirk-building.