The announcement that Matt Mullenweg laid out in his ‘Focus Tech and Design Leads‘ post in January 2017 is primarily responsible for the changes we are seeing to WordPress in version 5.
This post laid out three main focuses for 2017: REST API, the editor (Gutenberg), and the customizer. The new editing experience (which became Gutenberg) was mentioned in some detail:
The editor will endeavour to create a new page and post building experience that makes writing rich posts effortless, and has “blocks” to make it easy what today might take shortcodes, custom HTML, or “mystery meat” embed discovery.
This was followed just a few days later by ‘What makes a great editor?‘ by Joen Asmussen (a designer at Automattic) a post which opened up discussions about how ‘block-based’ editing would work in WordPress.