Mostly: timing.
Ember pre-2019 had a lot of papercuts (syntax, capabilities , etc) had just enough impact on the morale of developers learning React that React was a clear DX win (superficially).
Back then, ember was still more batteries included than any React framework (next existed back then, but usage was small), but it wasn't clear for folks who aren't good at maintaining software (most folks) that the future would become so bright.
Grass is always greener and all that. And Folks tend choose trendy things for side projects, things trialed on a weekend (this includes me, but i let knowledge cross pollinate in my projects between the ecosystems, it ended up making for some super solid architecture in the custom react frameworks i helped make). This is a demographic we're actually targeting with our upcoming edition, the Weekend Warrior.
Since 2019, i think ember has had better syntax and ergonomics than react and is only getting better 💪
But since it's been so long since people have used ember, if they even know about it, they perpetual lie that they believe about the framework and spread in socials, surveys, etc.
This is why i want
@StateOfJS to consider recency in usage for opinion, because we literally cannot treat what folks believe about anything, not just ember, with 5 year (and often 8 ) out of date information.
It is motivating to keep talking about the new stuff we have since then though (which is a lot).
Completely different framework now.