What about how crowded the arc is above both continents? At the beggining and obviously only C Band was used , the standard was 4 degree spacing , then 3 degrees and now we are 2 degreesby FCC. Therefore the need to use lower power TWTA's (well acctually for C band is also common to use another tecnology, SSPA) to comply with this spacing.
Guapoharry take for example ASTRA website and compare the TWTA values of those European birds with the SES NA satellites. Here in NA is usually around 50W , over there in Europe is around 100 with DBS services (they have to be 4.5 degrees apart in NA) having 50W more respectively.
The question is , Why the decision to still only use a reduced range of frecuencies as a general rule in NA then?. Maybe it is a historical one, who knows. Why not allowing another name of service (FSS 2 degrees, DBS 4.5) that uses the lower Ku band and therefore requieres Universal LNB's. There might be a boom in SAT services in the future , therefore there might be the need for the expansion of the frecuencies that are allowed to be used. Maybe right now , the Standard frecuencies are enough, therefore the need for a bigger Dish.