People laugh at the cute "Java runs on four billion devices!" tagline but back in 1995 if you weren't doing Java, you were a last-century coder. It was so pervasive Brendan Eich even renamed his strange scheme-smalltalk mashup "JavaScript".
(Also, Scheme-Self, not Scheme-Smalltalk.)
Twenty-five years ago
Hype is not reason why "a major Silicon Valley company that's migrating their backend to it".
I'm not bashing the language - it certainly has its defenders. Some people regard simplicity as a virtue (although I'd question how simple Java is nowadays). But I think even ardent Java fans will concede, Java has some deficiencies as a language. The main reason people use Java, is that people know Java. And the main reason they know Java, is because it is used.
It takes a long time to kill a network effect.
I suppose there are many Java enthusiasts on here.
I don't think that is true. Windows is still around.
Java had so many marketing crazes that it's crazy. I still remember the "Java mobile game" fad from a dozen of years ago - Java this, Java that, Java for phone, Java for toaster...
At some point, the public mind starts to associate "programming" with Java. Since there were (and probably always will be, unless we evolve into utopia) hordes of job-desperate people who would do anything to feed themselves and their families, a lot of them hear about Java, start programming in Java and develop their whole world around Java. Java helps them with this by providing this huge, Java-centric ecosystem which gives you everything you ever need to do whatever you want to do.
Of course, there is also the fact that Java is a managed environment and makes it a lot harder for programmers to write broken code, which makes large-scale programming possible even if all you have is a horde of mediocre programmers. But then again, that property is not unique to Java.