It didn't really take 20 years to build, as in they were working on it the whole time. They were building it and abandoned the project while it was still just the core structure, and left it abandoned for years, exposed to the weather. Then pictures of it, like the above picture, and articles about it started circulating around the Internet, saying it was abandoned and eventually would collapse.
Then the North Koreans noticed that we had noticed it. This really looked bad for the North Korean regime having an ugly, falling apart mega skyscraper hovering over their capital, so they decided to fix it, not by demolishing it or finishing it, but wrapping it in a glass shell so it looks finished. That might offer a bit of protection from the weather but it will still just sit there and rot away till it collapses. It has probably already been deteriorating too much to salvage.