Embraer vs. Airbus/Boeing on Short haul routes

Just thought of a good question, what makes an airline choose a RJ oriented company over a "major" company when it comes to short haul routes. Specifically a European airline, just to put into perspective the kind of routes being flown. If you are gettting high load factors (i.e. mid 80's or higher), what is keeping you from getting for example a A330 instead of a ERJ or CRJ for short/medium routes (i.e. London - Frankfurt). Some of the obvious reasons :1) Operating Costs2) Fuel Costs3) Cost of unused parts4) Pilot Training5) MX Training6) MAYBE FA TrainingAlso, some other reasons such as :1) Lost revenue from unsold seats (if you miss the upper 90's on more than one flight)2) More flights - More selection - More convinience for pax3) Less fuel being used (dependent on the number of heavies that would be bought)4) More Advertising for the AirlineMight have missed a few, but did I generally get the reasons right? I'm in Air Transport in college so I'm into the Business side of things now.