The reason is from the Engadget article.
https://9to5google.com/2018/03/30/google-url-shortener-shut-down/
https://www.engadget.com/2018/03/30/google-shutting-down-goo-gl-url-shortening-service/
https://9to5google.com/2018/03/30/google-url-shortener-shut-down/
The goo.gl link is very common on the web and was first launched by Google in 2009. However, the company announced today that it’s winding down the URL Shortener beginning next month, with a complete deprecation by next year. Fortunately, existing links will continue to work.
The URL shortener service launched in 2009 for FeedBurner and the Google Toolbar. With neither of those services available, the same is now happening to goo.gl for both consumers and developers. The latter group is being directed to Firebase Dynamic Links with today’s announcement meant to “refocus” Google’s efforts.
"many popular URL shortening services have emerged and the ways people find content on the Internet have also changed dramatically, from primarily desktop webpages to apps, mobile devices, home assistants, and more."
However, for average users that just want to truncate a link, there is no new alternative from the company, with Google suggesting Bitly and Ow.ly.
Starting on April 13, 2018, new and anonymous users will no longer be able to reach the goo.gl console to create short links. However, existing users will have access to all features like creation, management, and analytics until March 30, 2019, when the console will be discontinued.
Google notes “all links will continue to redirect to the intended destination” even after 2019, with users also able to export link information from the console.
Developers are encouraged to switch over to Firebase Dynamic Links that have the advantage of automatically detecting the user’s platform and send the user to either the web or your app.
"URL Shortener has been a great tool that we’re proud to have built. As we look towards the future, we’re excited about the possibilities of Firebase Dynamic Links, particularly when it comes to dynamic platform detection and links that survive the app installation process. We hope you are too!"
https://www.engadget.com/2018/03/30/google-shutting-down-goo-gl-url-shortening-service/
URL shorteners can be both useful and fun. Google's take on the tech launched in 2009, and added a third-party API, the ability to create QR codes and the power to link right to iOS and Android apps. Even Keanu Reeves has a URL shortener named in his honor. Unfortunately, Google is replacing its own service, goo.gl, with Firebase Dynamic Links (FDL) as of April 13th. These new smart URLs let you send folks to any location within iOS, Android or web apps.
You won't be able to create new goo.gl short links after the 13th, but existing users can manage them via the goo.gl console for the next year. After that, all the links will still work, but you won't be able to access the console itself after March 30th, 2019. Google suggests creating FDLs from now on, or using other shortening services like Bitly and Ow.ly.