My feeling is that they are not selling because they are not pushing them much. I don't think Samsung ever made a serious, competitive Windows Phone. Also Lumia is not selling the volume it's selling just because of the OS (actually for some is a disadvantage because of the smaller marketplace). It would be interesting to see how a Lumia with Android would fare.
Anyway, three years ago Samsung had 5% of the market and Nokia over 35%. They had a good lead over everyone no matter what OS they decided to back. Admittedly going with Android would have meant that they would have more competition, but I believe that they had a good chance on brand recognition alone. Perhaps it would have been worse, no way to tell now.
Nokia essentially dropped Symbian two years ago.
I think they have outsourced Symbian support. I am not sure anyone is developing for it or updating it any more.