My understanding from a talk back in 2014, is hack was developed because of frustrations with PHP evolution. Honestly I think it may have lit a fire under the PHP developers because php7 had significant speed improvements[4].
Talk:
"Facebook recently introduced and open-sourced Hack (https://code.facebook.com/posts/264544830379293/hack-a-new-p...), a gradually-typed programming language for HHVM that interoperates seamlessly with PHP. Hack builds the bridge between the dynamically and statically-typed worlds – providing code correctness while maintaining a fast feedback loop. Facebook is committed to working with the community to improve and refine the Hack language, to help interested developers convert to Hack (https://code.facebook.com/posts/264544830379293/hack-a-new-p...), and to narrow the HHVM compatibility gap with PHP5 and popular frameworks."
https://www.meetup.com/bostonphp/events/184609542/
Slidedeck on Hack (https://github.com/gabelevi/Boston-PHP-Meetup-Examples/blob/...)
[4] PHPs response to hack, phpng (php7): https://www.sitepoint.com/php-fights-hhvm-zephir-phpng/