They're actually all considered 6th gen consoles. There's only a 3 year gap between the Dreamcast and the Xbox.
Dreamcast was 98
PS2 was 2000
GameCube and Xbox were both 01, the year Dreamcast was discontinued.
Dreamcast could have been a wild success, probably would have been, too. The major issue was that the Playstation was still totally dominating the market. 98 and 99 were both ridiculously strong years for PSX title releases. Then the PS2 released and totally overshadowed it. Sega just couldn't keep up... Nobody could. Not until the market kinda leveled out in 05-06.