It's not about the price - as you note, RI is often priced the same or more as other brands.
And it's not about the breakfast - other free breakfast brands like fairfield earn 10x.
The three 5x brands - Residence Inn, Townplace, and Element - are the ones that Marriott views in the Extended Stay category. Maybe they feel that on a weeks-long stay you'll earn plenty even at 5x. Or maybe they think extended stay folks are less motivated by points.
And they may be right. I've had two stays in my life of over two months. In both cases, I opted for RI, even knowing the points were half, because for that long a stay, having the extra space and the kitchen was pretty important to me.