Most parameters of ejabberd can be reconfigured in real time, either using the web interface, ejabberdctl or ad-hoc commands. However, there are other parameters that can't be modified by any means.
The list of virtual hosts is read from ejabberd.cfg at startup, and there is not way to later add more vhosts, or disable existing ones without restarting ejabberd. This feature may seem easy to implement at first, but in reality requires several small changes in a lot of ejabberd core files. This also means that the debugging phase is a lot harder.
I guess the people more interested in such a feature is probably server hosters.