Because the provider only sets up running machines with 1 partition and 1 system on it. And when copying from one system to the other system while it's running, you can easily make mistakes that make the system unreachable. And thus abruptly interrupt the server update.
It would be so much easier for administrators if you simply installed 2 independent systems on the disks as a matter of principle, so that you could install into an inactive system during restructuring work. Well, the low price has to have some reason ...