Curious- base coat vs acrylic urehane for base

I admit I have painted all my cars since about '90
in acrylic urethane single stage. I did just one back in '90 is bc/cc system. I am no pro but I have painted a few dozen over the years . Here is what I am wondering.
Going to redo the wife/s car in '69 B3 Blue it as done 5-6 years ago, and I will seal it with epoxy primer, no body work and I want it to have clear coat. She likes more shiny.
I know it an be shot with acrylic urethane single stage and then cleared. Is not the typical 2 stage base coat a urethane plus reducer. Why would not the single stage urethane with its activator not be a more durable base? Is the 2 stage base just easier to lay with the metallic?
I know there are some pro body guys on here!!