iirc scala literally began as just java with pattern matching from odersky's frustration from building javac in raw java. pattern matching is well on its way in modern java, with switch expressions and instanceof binding in jdk14 preview and more general destructuring (further enhanced by records) and guards up next.
in reality though aside from the rock/hardplace of language conservatism/fanatical back-compat i have to imagine a real reason modern java has yet to fully embrace Optional/Either over null/exceptions is lack of value types - yeah escape analysis lets you ~mostly~ not have to worry about the IdentityObjects you're returning everywhere, until you hit something it can't/won't inline or try to stuff those Optionals on heap and wind up making our already painful pointer chasing situation ten times worse. value types are also well on their way via project valhalla but have yet to actually land.
maybe ironically one of the things i recently got bitten by was the fact that MethodHandle .invoke* methods throw Throwable when used directly in plain java - the very machinery that enables the efficient composition of these kinds of functional programming styles is gated behind having to catch literally anything in the language itself. i guess the recurring theme here is java putting the horse before the cart, and that's frankly my favorite thing about the ecosystem.
Checked exceptions were an excellent hack, but an ergonomic failure, despite the almost-pattern-matching of the `catch` clauses.