Why is the Wii U considered a flop?

At the time of its release, the manner on how it they’d marketed that was confusing as they showcased the Wii U gamepad as if its a portable console (its not) since its rather the main controller for the Wii U console. I was in high school when the console was first revealed, my reaction to that was “WTF?” as I thought the gamepad was portable, as the ads often flex that it can be (or the way they showed it came off as that).