The problem is that it's very inefficient and costly to operate very small numbers of an aircraft type. Training, including pilot certification and scheduling, cabin crew training (with most USA airlines requiring FAs to be qualified on all aircraft in the fleet, maintenance (including training), parts (including inventory costs at various hubs), etc. really become expensive, although the minumum number of matching aircraft needed to obtain most of the efficiently gains is actually very reasonable. In fact, in an earlier era, the number was documented to be about six in a study by the CAB that helped to support the move to deregulate airlines.