SQL Server (full ver) on the TRITON management server

What prevents using a full installation of SQL server 2008 R2 or 2012 R2 on the same server as the Management Server installation? All documentation says it can't be done, but gives no explanation as to the reason...

Is there a port conflict or other hard limiting factor that would preclude this configuration? It seems that if SQL 2008 R2 Express can be run on the same system, then a full installation could also be installed and run from the same system (given that the available hardware was substantial enough to host all the services on one box)...