Strange Question

Understanding I am not an airline executive who buys airplanes, I would guess the following factors, in the order shown, are key determinates in most aircraft purchases.

1. Suitability for the routes flown (you would not buy a B-737 for NY-Hong Kong routes nor would you buy a jumbo for short haul routes)
2. Size (seating capacity based on airline projections of demand)
3. Economic (the best combinations of purchase price/terms/operating effiecency)
4. Standardization (similar flight characteristics/cockpits instrumentation, etc)

Many airlines fly aircraft of different makes, however, not as many as in the past because there are fewer manufacturers of airline aircraft these days.

How I think they do it is by the amount of passengers. EX- If American Airlines usually has 50 passengers going to KLAX on flight 23, and they have 250 passengers on flight 53 going to KJFK, they are more likely to assign a 737 to flight 23 and a 777 to flight 53.

Sorry if it's confusing

I know for a fact that American Airlines flys MD-80s Boeing 757s and Fokker F-100s. I've been on all 3 makes.

I guess some airlines believe mixed fleet flying means when they are looking to buy they can negotiate a better deal ie play each manufacturer off against each other. If on the other hand you are only flying one type then the manufacturer knows you will be going back to them. Changing from Airbus to Boeing for example would be too costly for any airline to do overnight...training costs etc. The good thing that springs to mind with a single type fleet is that it makes life easy for crewing as all crew are type rated on the entire fleet, same for CC. Maintainance is simpler as is writing SOP's etc. Downside to single type is that if there is an airworthiness notice grounding a particular type and it's what you fly you are up a certain creek without a paddle!
I think this is one for the beancounters and not pilots!