He was already in talks for the CBS show Dr. Vegas when he met with the people making Grey’s Anatomy.
He loved that script, saying there was no comparison between the one for Grey’s and the one for Dr. Vegas.
So even though he liked the Grey’s script better, he went for Dr. Vegas (co-starring Amy Adams, btw) — which only taped 10 episodes and only aired five of them before it was cancelled.
Rob Lowe previously shared the story of how he turned down the McDreamy role in his 2011 memoir (via Us Weekly).