Mailing the story:

Per an announcement to the vim developers, Diego Viola who's interested in Neovim explains it as "to refactor and modernize the [vim] codebase." This was already criticized by vim's Bram Moolenaar who was quick to say, "It's going to be an awful lot of work, with the result that not all systems will be supported, new bugs introduced and what's the gain for the end user exactly? Total refactoring is not a solution. It's much better to improve what we have. Perhaps with some small refactorings specifically aimed at making Vim work better for users."