By trying to support too much hardware at once...
Sonic Team was busy developing S3&K since that's where the money was in the NA market at the time. Early Saturn game development began at the end of '93 (Panzer Dragoon), but S&K wasn't released until the end of '94. There was no way they could have made a launch title for the Saturn. NiGHTS wouldn't even come out until the middle of '96.
On top of that, SOA had shifted its attention to the 32X. Sonic X-Treme started out as a 32X title. That didn't last long. It was all too unfocused.
The Dreamcast got a lot of things right - one of them being a Sonic launch title.
There are a lot of factors related to this (see all of Melf's research), but it certainly seems that SOA's primary concern was marketability. And that's certainly where you want to focus to boost your market share, although I have no doubt that it hurts the legacy of the product. In Japan, Sega president/autocrat Hayao Nakayama was much more focused on creating fun/innovative games (Treasure's president Maegawa has attested to the incredible freedom they got to make whatever they wanted) - and then basically not marketing them. The contrast is striking.