What does Sela blame Picard for?
Well, her defeat in Redemption.
Her defeat in Reunification.
It could have been explained she lost a lot of status and was disgraced, so she was banished to Remus, where she raised a Reman army for a coup. Picard is dispatched, and she is bent on destroying her long time enemy. No less believable than Shinzon.
...and, most importantly, and it is twisted motive to be sure, for her being born. Remember, it was already established she was ashamed of being half-human, and hated Picard for sending Yar back in time, where she became her mother.
I'm not saying Sela didn't have motive for wanting revenge. All I'm saying is that Sela's motive would have been a hell of a lot more complicated to explain to a feature film audience than Khan's, which is probably part of why Khan works so well as a villain and Shinzon/Sela didn't/wouldn't.