> custom language
Vala wasn't a custom language for elementaryOS, it already existed for years before the first version of the OS (and i think the first version didn't use any Vala code).
It most likely used instead of Objective-C because its object system was built on top of glib's gobject system and elementaryOS used Gtk and a lot of other glib/gobject-based libraries. Using Objective-C would mean they'd have to base their UI on GNUstep and GNUstep wasn't as polished as Gtk.
Remember that while they used macOS as an inspiration, it was never elementaryOS's goal to make a macOS clone.