There are 3 elements that sell hardware: Price, Games & Graphics, you could also throw in Gimmicks into the equation as well if you want.
Good games need nice graphics, they don't need to be brilliant graphics, but nice. Bad graphics can harm a good game though.
The Wii doesn't have bad graphics. They are good graphics although they are not up to the standard of the 360 or the PS3, they are still pretty good.
IMO, the Wii has sold well because of mainly 2 reasons. Price and Gimmicks, the price being much lower than any other comparable console and gimmick being the whole Wii thing as a whole. Yes the Wii will have good games, but Nintendo rely on price and their good olf favourite Mario to sell their systems.
I feel that Nintendo intentionally made the Wii to a lower spec so that they could place it in the market at a significantly lower price than the competition especially the PS3.
You will always get games without great graphics being brilliant games, but gamers are looking for more for their bucks as well and graphics in the end sell games.
Prediction (June 12th 2017)
Permanent pricedrop for both PS4 Slim and PS4 Pro in October.
PS4 Slim $249 (October 2017)
PS4 Pro $349 (October 2017)