[zimbatm]

Building on top of other people's infrastructure is expensive.

Each node can disappear at any time and is really hard to predict. To compensate for that you will need to upload your data to more nodes. You have to deal with backward and forward-compatible updates because both client and server versions updates are also not under your control. There are also more complicated issues of trust and bad actors that can affect the product.

Now you have a datacenter. The clients upload their data exactly once. Your server software has at most two concurrent versions during deployment. Security is handled in a traditional fashion with DDoS firewalls and pentesting. Your whole software stack is much simpler because you can build on top of more solid assumptions.

Skype also used to be P2P until the Microsoft acquisition where they moved towards hosted nodes. Some people say it's so that Microsoft can spy on everybody but I really believe that the main reason was so that they could improve the QoS.