This could be a simple answer, but in the last few days, there are just a few topics which have popped up in the Civil Aviation forum
"DL orders 2 more 777s" ... quite a costly figure to just "order" two more ...
"Junpeng Buys 100 ARJ21"
"Two More Ex-AA/TW 757s For DL"
"Saudi Arabian Dumps MD-90s, Buys Airbuses"
"Grupo Marsans Signs For 2 More A380 Options"
"Tiger Airways Orders 20 A320" (meanwhile some are being scrapped)
"CO's 737-900ER's Features + Delivery"
these are from the Airbus website
"Spirit orders 30 A319s " (Airbus website)
"Jazeera Airways orders six more Airbus A320s "(Airbus website)
"Yemenia orders ten Airbus A350 XWBs " (Airbus website) , i find these humorous, they just invested how much into an aircraft which has not even taken off yet?
For an airline to just buy 100 aircraft, or purchase 3 - 10 aircraft which are priced at 250 million dollars per unit, seems like a very VERY costly investment. If American in time replaced all their MD80 series aircraft with 737s, what would happen if one route became less popular and was not worth flying given the expenses involved?..that would be one 737 sitting around. I am sure it's not people with the IQ of George W Bush making these calls, but what factors come into play when huge quantity or pricey orders come into play? (apply this example to any airline).
In other words, you own / have leased to many aircraft to suit your needs. Does Emirates seriously need 58 A380S? Have some Airlines been buying and leasing on impulse rather than demand.? TAM for example just acquired both of ACs 345s... for how long was it said in these forums they were going to Ethiad or Singapore?
I am sure there are many factors out there, so lets toss some out.