Cost savings of operating the 42 compared to the 72 are as follows;
Lower ground handling costs,
Lower landing fees at some airports especially regionals,
Half the cabin crew costs,
There will be a fuel saving albeit not a significant amount but still a saving,
A lot of operators favour the 72 because with higher pax loads all of the above costs are then accounted for with additional revenue whereas if you are averaging 30 pax on a particular route, the 42 is the obvious answer.