Because in relation to what video games were back in those days and what they are today, they were not as underpowered as the Switch is.
Also people change, my demands now are not what they were 10 years ago. 10 years ago gaming happened on TV's whose definition ranged from low def (480i) to high def (1080p) and my Wii while on the low end was still outputting within that range. Today the norm is between 1080p and 4K and a game like Zelda is NOT in that norm, it outputs at 900p if I remember correctly with a lot of aliasing.
I totally get that the Switch is doing great considering it's mostly a portable system but like I said previously I don't have to suffer the limitations of a portable system when a dedicated home system could have done the job and like I said people change, 10 years ago I was not thinking in terms of dedicated home systems, now I am.
Hm, I actually feel as though the Wii was more underpowered for it's time than the Switch is. When it comes to the Wii, proper cables are a major necessity if you want to play it on a modern TV. I bought the Wii on launch day, and I had a fairly new HDTV at the time, and with the default cables that came with the box, it made everything look gross on my TV. In fact, I actually had no idea that better cables existed for several years, so I always thought that that was just the way games looked on the system.
The Wii has some awesome games, but in my opinion it's by far the worst console Nintendo has ever made.