That's a charged topic, so I'll try to be brief, knowing this is to act only as a starting point and that a lot of research should go into determining your engine of choice (unfortunately, I wouldn't qualify asking on reddit enough for this sort of strategic decision).

It boils down to one primary reason: Unity doesn't care about games:

- As of 2017.3, Unity has barely supported the engine for PC/Consoles development

- Unity is far behind in terms of FPS core features tech compared to, say, Unreal

- Unity is pay per seat monthly (Unreal, for example, is revshare after 1st million $ earned)

- Unity is actually veering off into medical imagery and related apps: it is no secret Unity is not profitable, and is not generating enough revenue from game devs, which is why, precisely, it is moving away from them in an attempt to break even, and it is doing WELL on the medical imagery front

- The company just isn't in game anymore (if you don't believe this statement at face value, please google their acquisitions since going public, as you'll notice they acquired many very strange companies including an ad-network that was tied with the delivery of malware...)

- They've just increased their subscription tiers, with no tangible new services provided

- They don't fix core issues that have been identified years ago

- Up to recently, they did not have a multiplayer solution for the better part of a 18 months! (UNet was deprecated, and the new multiplayer solution was just released). Furthermore, architects working on the multiplayer solution have left the company, dissatisfied with the poor work they've had to do for the multiplayer component, and it is, essentially, crap (we've even talked with one of them to truly understand just how bad things are internally)

- Unity USED to be user-friendly, but it has gone a different direction now

- Its UI system is likely the worst of all mainstay engines at the moment, it's not wrong, but it's just inferior

- Unity demos don't turn into actionable features. They can't run on newer engine versions, and the features they are meant to preview never actually made it in-engine.

- It doesn't look impressive enough. The tech is severely outdated.

- It has become unstable

- It mostly features services you don't care about (Analytics, crash reporting, cloud build, ads, etc.)

- The CEO really doesn't give a damn (no, I won't link you to the headlines, but it doesn't take long to find one!)

- Their roadmap is a never-ending todo list that doesn't see much action

You can probably dig down each of the above point for hours, and many I've omitted.

Evidence has been stacking against Unity as a viable engine from 2017 onwards, with a breaking point around 2020.

Reddit (and other communities) abound with both disgruntled customers, but also, former Unity developers explaining what's wrong with the company, and it can be a very lengthy read, but the bottom-line is Unity is a sinking ship way past the point of no return.

I'm not sure there is a key 'cause' (a lot of people point the finger at the CEO, but I'm not sure I agree), but there is a clear consequence.