Chiming in to say exactly this. Safeco has the luxury of sitting beside a bunch of railway tracks, so they basically build a normal ballpark and just suspend the roof over the tracks when it's nice out, and then slide it over when it's not. Most other ballparks don't have that much free space to work with, I don't think. It's an awesome park and it's an unreal experience every year when the Jays are there. Can't wait for this year's trip!
http://www.ballparks.com/baseball/american/seabpk70.jpg