Boeing 767-400's

How come the Boeing 767-200’s and
767-300’s sell extremely well whilst the 400 series has only managed to break sales to only two airlines:
Continental Airlines
Delta airlines

Also the same problim occurs with the 717 as it is in operation with only 6 airlines:
Air Tran
Hawaian
Veulamex
Bankok Air
Olympic aviation
Qantas Airlink

Is there some sort of problem with the aircraft