Facebook like Google is really inconsistent when it comes to supporting new features like dark mode. Both Telegram and Signal support dark mode. WhatsApp's resources and development team are probably far larger than either Telegram or Signal.
I haven't used Facebook for about 3 years now, but from what I can remember, the Messenger app was always first to get new features. I don't know whether it is because the Facebook app is massive with an old code base and maybe WhatsApp is poorly structured or designed which makes changing to things like dark mode a significant undertaking. Facebook Messenger came after Facebook and WhatsApp so they probably had a chance to architect the app in a better way which made future upgrades easier.
I am still waiting for WhatsApp to support the iPad. I have given up waiting now and try to get friends to use Telegram or iMessage instead.