most likely because of bandwidth and latency, if you call using wa we fb viber, the calls could be centralized(they didn't mention so I don't know as well) and if it is centralized, bandwidth and latency would be an issue as the data needs to be transferred to another country(international bandwidth costs more money, and sending data is another country and getting it back is not so good for latency sensitive jobs like voice calls).

Even if it is not(runs on p2p like Skype), to find your other partner it would use something like a hostname to find it where dns lookup would be required, port knocking if you're on a firewall and a lot more which may slow down the connection.

With Wi-Fi calling, they can limit the calls to use bandwidth locally (pay less because don't need to pay for international bandwidth.......... while still charging you the same ), better calls due to route optimization, and less latency(potentially, due to ip based connectivity(since it is inside the network so private ip can be used))

Anyway, take it with a pinch of salt, I'm not a networking guy