An educated guess would be that the airspace was being used at the time.
You can fit many aircraft in a piece of airspace at the same time. You can funnel them through, they can change speeds to better allow transit, delay, provide vectoring, hold or otherwise, different altitudes, VFR and IFR... like I said, it’s not like it’s special VFR where only one can be there at any one time. If it’s too much workload for one controller, then there is a problem with the system, split it up and find another controller. It’s not rocket science!
Yes it is up to the pilot to turn back, but forcing them to take alternate routes down lower, possibly in turbulence, closer to cloud, over tiger country,
or perhaps precipitation is not ideal and carries a greater degree of risk. Why take the backroads when there’s an 8 lane freeway available.... but you can’t, because there’s another car on it.
Don’t just accept the status quo because it’s always been that way. Stir things up to make a change for better for all of us!