When a switch is selected as root bridge why are all ports on that switch in forward mode? I know switches that are connected to each other by two ports have one blocked port and one forward so theres only one why for frames to go so no loops are made.
Because for most end users with a single switch, they want all to forward I would say. If you are at the point where you need to interconnect switches by multiple ports, you know what STP is for and why you should use it.
As per my understand you would like to know about 'why root bridges ports are in forwarding mode?"
My Answer: Root bridge is the master in spanning tree topology. So all decisions like timer changes and port states will be decided here. And any changes happened to the timers or topology changes must be communicated to all nodes which are participating in STP. And nodes will apply changes only if the communication is from root. So root switch's ports will be in forwarding state.
Hope this solves your doubt. Still if you have any thing to ask please update the question.