U.N. Owen
In Brightest Day, In Blackest Night ...
To actually answer the thread question, here are some possibilities:
- The writers/executives/whoever are done with Serena as a character. They're using the popularity from XY to market more dolls of her, controlling her appearance in media in order to prevent anything that might be unpopular and damage the brand.
- They have plans, but are waiting for the right time to strike. After all, absence makes the hearts of the fans and the weirdos who self-insert themselves int Ash and/or Serena grow fonder.
- The end is suppose to be open-ended just as a means of testing shelf-life of her character. Who knows? Maybe down the line, Serena gets backlash or a new pokegirl gets popular because of a wonderfully sad backstory and good characterization.