As with most things, it was probably a money issue. She was quite a bit higher profile than Torri Higginson at the time.
That seems to be the case with Atlantis in general. I think Mallozzi wrote a couple of months ago in his SGA walkthrough that a high profile actor was in the running for the role of Sheppard (not Ben Browder, another) but they decided to hire Joe Flanigan instead. And he's not exactly God's gift to acting.