You remember that scene in Days of Thunder where Cole and Rowdy drove to dinner in two rental cars? They were supposed to drive there together, in the same car but racers being racers they couldn't leave well enough alone. Their options were to suck it up or do something different. Their choice ended up being to rent a second car and then beat the ever living crap out of both cars as they raced to dinner. Sure that was a NASCAR movie but the point is if there is an opportunity to do so, racers are going to race. The competitive fire is too great to overcome.
The event was way back in 1993 and Senna was so competitive that he had the organizers ship a kart to Brazil so he could practice. Let that sink in, this was supposed to be a fun event to give fans a glimpse of something different than they were used to.
Senna and Prost weren't the only big names in attendance. Damon Hill, Johnny Herbert, and Olivier Panis all took part in the event. Prost had won his fourth and final World Championship earlier that year afterwards he promptly retired. This little kart race ended up being the last time Senna would race against Prost.