I suspect the reason is that it is actually quite complicated functionality that is required in a booking module. The basics are quite straightforward, but it quickly gets to be hard to have a system that can cope with all the possibilities with varying room rates, amendments to bookings, confirmations, ensuring that bookings made elsewhere are included in the system etc

it is available in Joomla because a Joomla developer wrote it, or at least a company that has chosen to use Joomla.  There is absolutely no reason that a comparable module does not exist as a DotNetNuke module except for that simple fact that no one has written one yet.  There are no limitations in the DotNetNuke framework or APIs that would prevent such a module ( or possibly even a better one ) from being written.