IIRC Skype peer-to-peer architecture was a very bad fit for smartphones. For instance text messages in Skype were sent directly from client to client, requiring the two PCs to be up and running at the same time for the communication to happen. It was brilliant for PCs running 10 hours a day, making Skype infrastructure very light.
Now take two iOS smartphones, where Skype will only run when the user is focused on the app. The peer-to-peer link was simply not going to happen. Skype brilliant peer-to-peer architecture simply didn't work anymore. This is why Skype was so late to the party.
Skype went from peer-to-peer to centralised right after Microsoft bought them out - this was before modern smartphones got popular - and to my knowledge the smartphone apps were always centralised.
Now take two iOS smartphones, where Skype will only run when the user is focused on the app. The peer-to-peer link was simply not going to happen. Skype brilliant peer-to-peer architecture simply didn't work anymore. This is why Skype was so late to the party.