Mostly due to very poor marketing,most did not even know it existed.
In the U.S., it was HEAVILY marketed. To this day, 9•9•99 is synonymous with "It's thinking" (for me at least). Print and TV ads were rampant, at least. At the time, the launch was reported to be the biggest single day ever In terms of entertainment revenue. Beat out any movie, sporting event, album sale, etc. up to that point.
And I wasn't even a Sega fan at all. I only got a Dreamcast because the PS2 was always sold out and I got tired of coming home with nothing.