Her departure from the show was so sudden and abrupt, like they weren't expecting it.

http://blogs.pitch.com/wayward/FamTiesalexellen.jpg

The show wanted her to stay, but she wanted to leave. She was commuting from LA to NY every week. She was in a relationship with a guy (not MJF) at the time and wanted to be with him. I believe the flying back and forth every weekend became too much and she decided to leave the show. The writers as a result had to come up with an explanation of her character's Ellen leaving. The character got a dance scholarship and left to go to Paris.