What were the main reasons for it not succeeding against the newer competition?
Cost of operation -vs- Canadair and Embraer. Seat mile costs secondary to actual aircraft costs on the operations the US airlines use them on. They just wanted a small cheap jet instead of 30-50 seat turboprops.
Maintenance expense and dispatch unreliability of 4 engines.
Not a good overall reliability record.
Cabin not quite wide enough for 6-across seating in the US market (they are fatter than Europeans - don't laugh, it's true). 5-across seating for the US market lost any seat mile advantage.
Noise and short field issues only relevant at a very few destinations.
US pilot scope clauses (146 mainline, RJs outside).
By end 2001 no orders (apart from British European) for the RJX and no prospects of any meaningful ones.