IMG probably don't have the resources for multiple respins of a design, so they'd need what they refer to as a lead partner to help push a new core timely through the final stages of bug fixing and validation.
SEGA probably couldn't justify taking on that extra expense for just arcade technology, so they'd have left the responsibility to IMG who'd have to work through the design by hand. The timetable, then, for delivery of the core probably slipped too much, and SEGA ended up going to nVidia and already-established set-top level graphics parts.
