Basically Solaris was the first Unix to make it cheap to have lots and lots of threads constantly being spawned and destroyed. But this optimization did them no good with existing C and C++ programs which didn't use the feature. But with Java it did. And even more in the early days when all I/O was blocking so you had no choice about spawning new threads at every turn.

In the defense of competing systems, the POSIX standard for threads was released in 1995 and Java was released in 1996. So they can hardly be blamed for not having optimized for that then uncommon use case. But by the time they fixed that, Solaris had already taken a solid market lead. Which helped Sun become "the dot in dot com".