The Dreamcast was because everyone was waiting for the PS2, also because they misunderstood how the market changed since the PS1. Ironically, Sony presumed people would wait for the PS3 in the same way failing to see how the market had changed since the xbox.
It is a mistake you see time and time again in the console industry and no console has held the top spot for more than 2 generations in a row. However, Nintendo took their beating with the Gamecube and came back strong with the Wii, this time with the idea that the market had changed to one they could not compete in anymore and so decided to explore a new market and its worked wonders for them.
Again, In another twist of fate it was Sonys PSP and its attempts to dominate the handheld market that forced the Nintendo to find a way to compete other than power which led to the DS, to have a console that
played different. By accident it found a new market in areas nintendo did not envision when they created and they took this approach to the Wii.
Basically its gone full circle, the market is back where it was before the power spec race. To when gameplay was king
Shame SEGA didnt adapt.