I have a guess - I suspect it's because in the domain it addresses, attitudes are towards self-reliance, privacy and autonomy.

If someone uses Tailscale in some cloudy (aka all someone else's computers) setup, they probably don't bother. They already shift trust and rely on other people.

But Tailscale is infrequently used to manage own devices, which is why it clashes with self-reliance attitudes. If you run your own private hardware and networks, all or many of those in your own castle, it may bug you to introduce (or I'd rather say trust) a third party unless you're forced to. Not because it's not trustworthy, but because of the self-reliance attitude and desire to have full control over what you think of your own systems.

Sure, you're forced to trust your electricity provider (but you have an UPS) or network uplink (and even then you make security precaution), but trust in Tailscale is kind of optional (it's not irreplaceable), and not everyone feel like they want it.

Pretty much the same why a lot of people frown upon IoT stuff, even if it's from rare vendors who go extra mile to make things reliable - because of the hypothetical "but what if?" and feeling that you're losing the control here.

Just a guess, though. Others' mileage may vary.