The new boss bar API supports legacy text only, even though the underlying system is entirely component based. It seems silly to be implementing new APIs using functionality that has been deprecated for years now. If the Bukkit API is ever going to have component support, then surely there is no better time than now to add it.

Spigot's component support is just fine, so it's just a matter of porting it upstream, as far as I know. SportBukkit expands on the Spigot support a bit, including boss bars.