Amazon, rather infamously, nuked Comixology's browser site 2 years ago and now they're completing the job by phasing out the dedicated app in favor of the Kindle App.
"Today, we are announcing to customers that we are merging the Comixology app into the Kindle app on iOS, Android, and Fire OS. Currently, 93% of comics, graphic novel, and manga customers are reading via Kindle and the Kindle app, and Kindle reading is where a majority of our customers have been for years. The upgraded Kindle app experience is already available and has key Comixology features for enjoying a digital library of comics, graphic novels, and manga, including grouping titles by series; hiding and unhiding specific titles; library filtering for comics content; in-app borrowing for users of our unlimited subscription services; and additional options to read on the web. Customers can continue reading their books in the Comixology app until December 4. Focusing on the Kindle app improves our customer experience and advances our mission to make everyone on the planet a life-long comics, graphic novel, or manga fan."