According to a post on the official forum, DTG's team (and management) doesn't "[...] see a clear direction that will allow us to keep to the development time we’d want, alongside the player numbers [...]"
The team also published a FAQ:
What happens to Flight Sim World if I already own it?
It will remain in your Steam library, available to play in its current format.
Will the sim be taken off sale?
Yes, and we’re working to the date of 24th May 2018. We want to give anyone who does not yet own a copy of the sim enough time to get a copy and keep it safe in their library for future play.
What will happen to any Add-Ons?
Add-Ons will also come off sale, but those you already own will still also be in your Steam library.
We will surely see a lot of "I told you so" posts out there but we should all respect the team's work and effort in making a successor for FSX, and a new and better sim to our community.
Losing this projects is never a good thing.