Another design consideration which isn't as relevant to the 737 configuration, but I'll mention it anyway... on rear engined types, you don't want to have inflight spoiler type devices in front of the engines, for inlet distortion considerations. That leaves a big area of the inboard wing, that you'd like to be able to spoil for the landing/RTO cases, but want to avoid inflight. That naturally leads to installing ground-only spoilers in that location.
Even for an underwing type, spoiling the inboard wing in flight has more effect on the downwash at the tailplane than spoiling the flow further outboard, but that's not as big a factor as the engines. It's still a reason to prefer to not spoil the inboard flow, though. (There is a countering consideration, on swept wings, that you don't want to go too far outboard with flight spoilers for risk of having unacceptable pitching behaviour. So the mid span is where you tend to end up, for this and other reasons)