It got introduced at time when completion was less fierce and competing languages sucked at that time. Sometimes it's just as well about timing. Swift, Kotlin, Typescript, Rust didn't exist at that time. If today Go got introduced for the first time I doubt it would gain any momentum.
Swift is probsbly to late to the party considering big competition amount languages and their ecosystems being "nice" today is not enough.
Being backed by Google was huge. Swift is backed by Apple but not outside of their specific use cases. If Apple had announced they were going to start making all their backend services in Swift too I imagine things could have looked very different.
* linux, windows and wasm compilation targets. They hired the maintainer of wasm project as well.
* LSP language server for Swift, to power VSCode and others editors, even though, they use Xcode that doesn't need it
* well tested server side libraries similar to netty/jetty (they even have a former Akka developer)
Other commenters have gone over many of the reasons why Go got popular, and I don't claim to know for certain, but in my view one important one is probably Rob Pike. He obviously has a well-known pedigree, he's very articulate, and a lot of people latched on to his "Go philosophy". Combine that with the reputation for unrivaled engineering excellence that Google had in 2009 and it's not surprising that Go got as big as it did.
Google hype, popular cloud native open source projects showing its potential, well known creators, good to acceptable tooling, strong standard library, low memory footprint, concurrency, simplicity, productivity, the whole philosophy behind it.
Some of these are questionable, but I heard all these by some people.
What does this say about Swift's chances to gain mainstream popularity outside of the Apple ecosystem? Probably nothing, as the languages and the circumstances are so different.
Go drove the cloud-native migration from java/enterprise, riding the wave from behemoth machines to small containers, as a result of Google deciding to build alternatives to Java/Oracle and raw C.
The ease of the language and the self-contained binaries just changed how fast that could happen, not whether it would happen.
