It's tied to the adventure mode.
One of the main differences about Team Sonic Racing compared to the previous all-star racing outing is the fact the cast is limited to the Sonic universe. Characters from Sega's other games simply didn't make the cut this time around.
If you've been wondering why Sumo Digital dropped characters like AiAi from Super Monkey Ball or Ulala from Space Channel 5, in an interview with GameSpot at the start of this month, Sumo Digital's studio design director Derek Littlewood explained how this decision was tied to the team adventure mode and wanting to explore the Sonic universe in more depth:
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