Probably the usual reasons of pay and other Ts and Cs.. i suppose. Maybe they'd like to work out of Heathrow or Gatwick or wear a Julien MacDonald designed uniform. There's always a mulitude of reasons for people moving from one airline to another. I'm not sure everyone's only goal is to gain a command as soon as possible.
Edit: Your edit makes my post a bit redundant now... ho hum.
And again, cos i said Ts and Ps instead of Ts and Cs...oops