It's the Allard J2X-C; A Group C car built in 1991 for the new rules that overhauled Group C and transformed the class into a "Sprint" series with cars built around F1-Spec 3.5 Litre, Normally-Aspirated Engines (When it had been a series with virtually no limitations on engine type, but severe limits on fuel consumption)... Though the rules only last two years (1991 & 1992) and ultimately killed off professional sportscar racing in Europe (For a short while at least!), some of the most advanced Sports Racing Cars ever built came from that series, the Peugot 905 being the only example in GT5 or GT6 at the moment...
The Full Story of the Allard can be read here:
http://www.mulsannescorner.com/allardj2x.html
As for WHY those new rules came into effect (Especially when the "Old" Group C class and the World Sportscar Championship were thriving at the time), well... It's complicated and this thread's not really the place to discuss it in my opinion.