It's all about the kind of air mover. What you've linked is a traditional fan which blows air front to back. What you typically see on the end mounted systems are blowers. They take air in (in the center of the "fan" part, from the opening you see in the card, and blow it out radially. Usually they're shrouded along the back to push air in one direction (across the components and out the case). For a blower, it makes sense to mount at the end, and send air across the whole card out the back. Since a fan is blowing air down on the card (as opposed to across the card like a blower will), it needs to be in the center so that it can spread both directions. You can't really duct a fan blowing down to make a 90 degree turn and have enough force to move a large amount of constrained air across the length of the card and out the back. However, for the blower, this is perfect because the air is propelled parallel to the card anyway, not perpendicular (and then bounced off) like it is with a fan.