A318/19/20/21/30/40. B717/27/37/47/57/67/77/87. CRJ2/7. ERJ145/175/190. FKR50. IL62. MD11/82/83/88. TU154. AA AC AF AI AR AS AT AV AZ BA BW BY CO DA DL F9 FR JJ KL LH MA NW NZ OS RG SU TK U2 UA US VS WN
Another reason might be because they don't have the equipment or qualifications for it, or they're not authorized to fly that route. The latter can be the case with international routes where foreign governments have something to say about it.
Or it could simply be because they can't get slots at the proposed destination airport. A lot of airports are slot restricted and getting slots isn't easy. If they could get slots it would no doubt be profitable, but that is a big if.
Unfortunately, there are still many folks on A.net who are either unable to process thoughts or unable to reason logically on their own (or make the time and effort to conduct some research) and they will continue to ask these questions. Nice try, however.