It's important to realize that despite what some people here think, Marriott's core business is being a hotel franchiser. While they do have a side business that provides hotel management services, the main "customer" for Marriott is the hotel owner.
The points that hotel guests receive are paid by the individual hotel. Complete speculation, but I'd imagine the 5 points/dollar anomaly can into existence due to way things were at the birth of the extended-stay market segment. Perhaps back in the days of yore when the rewards program was conceived, the profit margin at the extended stay brand (only Residence Inn at that point, I believe) was slim enough to warrant fewer points. (That assumes some brands always had a lower earning. I don't know my frequent guest history enough to know exactly when the anomaly started.)
Anyway, my point is that you'd need to look at market conditions back then and not today for the reason why these brands offer fewer points per dollar. I don't think these brands are significantly different today to justify the lower point earnings. But, once something that benefits a franchise is part of a franchise agreement, it's hard to remove it. If neither the franchises nor Marriott see a compelling business reason to switch from 5 to 10 points/dollar, expect the status quo to be maintained.