That's comparing apples to oranges.
With the supplementing the , the affected area was between Manhattan and Brooklyn (namely Brooklyn Bridge and Atlantic Avenue). It makes sense in that case to send a between those two stops, as folks can transfer to the to connect between those two stops.
With the suspension, the affected area is within Brooklyn, between Church Avenue and West 8th Street. The is either within walking distance or a quick bus ride away from most stops in that section, so it makes sense to redirect them there. Running it local allows for a connection at 4th - 9th, which means that those customers between Church Avenue and West 8th now have easier access to stops in Park Slope and Gowanus, rather than having to backtrack to Downtown Brooklyn or further north just to go back down.
Extending the and down to 9th Avenue does nothing for those folks, because they don't need a connection from Manhattan to Brooklyn. They need service within Brooklyn. It would really only work if those trains ran further south, which isn't feasible given the current crew shortage.