Managed and unmanaged hosting can be divided on the basis of scale of support they are consuming from web hosting providing companies. The number of customer for unmanaged hosting are less than the managed server hosting following are few reasons for it.
In case of managed managed web hosting solutions, web hosting companies provide support and service almost for total hosting package. There are some exemptions for softwares and applications which a client want to install in his own behalf. Such softwares mostly contains applications like wordpress or drupal and so on. But the major parts like operating system, cPanel, or setting up a server are performed by support staff of a web hosting company, this type of support also includes a few applications which need to be installed before hand over to client. There are few host who provides some special services to their clients such as automated back up facility and few tools which monitors server for providing alerts for any possible downtime.
When things come to unmanged hosting services, generally the clients are tech savvy and are capable of managing servers at their own risk, so they only need for hardware. Many tasks such as replacing any physical component, maintaining network to keep server on all the time and rebooting whenever demanded by client comes under hardware support. The risk factor shift more towards webmaster in unmanaged hosting services as maintaining all software and other applications is sole responsibility of webmasters and not of the support staff of web hosting services provider.