Original response below:
That's really a WhatsApp product issue, not an OS issue. There's some hints of an OS issue, because Android lets WA put a backup file on the 'sd card' that you can transfer across to a new (Android) phone, and iOS doesn't (or didn't), and with cloud backups the different OSes both tie into their own clouds.
But the main issue is the WA iOS app and the WA Android app have different schemas for their on device database, which makes it not so easy to move. Maybe that has changed since I stopped working there, but that was the biggest issue with a switch platforms feature that I was aware of. It's a lot of coordination for a feature that most users are never going to use, and if they do use, likely aren't going to use it more than once. When I recently got a new Android, I did see there's a new transfer data flow for at least Android to Android, so maybe there's hope for cross OS data exchange in the future? It's also helpful that there's only two relevant platforms now, instead of 7 (s40, s60, blackberry, blackberry 10, windows phone are all dead)