Two things killed the Dreamcast:
a) The Saturn - this was a money pit that caused Sega to post huge losses, and gamers looked at the Dreamcast and figured it’d be the same, so they didn’t buy it.
b) The PS2 - After looking at the Dreamcast, then they see that a much better(debate this if you like, but that’s the way it was perceived, at least) console was around the corner. Sega didn’t stand a chance.