Yeah it looks nice but the price is that it's broken everywhere else and will probably be overtaken by newer engines until it reaches a release. That is...if it ever does.

Otherwise, not many games made it to release on that engine: https://en.wikipedia.org/wiki/Amazon_Lumberyard

It’s a pretty new engine (it’s still beta) and is being used by Amazon Game Studios mostly:

https://amazongames.com/games/index.html

Lumberyard is targeted by MMO's due to Amazon's content delivery network abilities... and MMO's are the biggest genre to fail before during and after release, purely because of their scope and difficulty to pull it off "right".

Lumberyard is a fork of the CryEngine, and quite a few games have shipped on either CryEngine or one of its forks (particularly Dunia, which Ubisoft uses extensively).

So what is the reason the Lumberyard fork is so "unpopular"?

I wouldn't say unpopular, just relatively new. The only games I can think of running that engine haven't released yet -- RSI's Star Citizen and Amazon's Crucible. Star Citizen's long development cycle and various funding sources have been well-documented, and it still lacks a concrete release date; on the other hand, Crucible comes out next week.

Because it’s new?