You also have more reverse proxys and routers handing traffic now, so someone attempting to access ftp services would be routed to the ftp server, and someone trying to access http resources would be routed to that server.
Routing can be handled at the protocol/port level instead of the subdomain level to make connections a little more user friendly.
I think it is also related to computers becoming faster as well. In the old times ftp.example.com, mail.example.com and www.example.com could be run from different machines.
You also have more reverse proxys and routers handing traffic now, so someone attempting to access ftp services would be routed to the ftp server, and someone trying to access http resources would be routed to that server.
Routing can be handled at the protocol/port level instead of the subdomain level to make connections a little more user friendly.