Which is easier?
Purchasing a server, hiring someone to set up your email service and domain name. Setting up email accounts and passwords and administrative passwords, hiring someone to maintain your server, purchasing additional hardware, transferring the emails from one server to the next, on multiple occasions, performing software upgrades, maintaining Internet security (stop laughing!), maintaining physical security, paying for power for servers, maintaining temperature of server room, etc, for four years...
Asking the State Department for a (free) email address and choosing a password?
And Hillary said, she did it for her "convenience". (Plus the Clintons are notoriously cheap. So, draw your own conclusions.)