Because the car and driver models were finalised by the Spanish Grand Prix in May. Yes, I’m aware that it doesn’t require anything more than a quick palette swap for the helmet and a name change, but there is still the need to code a system that would allow for a driver change, and by that time, Codemasters would have move onto more-pressing matters. Because they’re releasing the games in yearly installements, the development schedule would have been very tight, and so called for all the car modelling to be done as soon as possible because Codemasters had to develop two circuits (the Nurbrugring and Buddh) from the ground up, work out the gameplay mechanic for DRS, introduce the safety car (which apparently caused a lot of problems) and probably half a dozen other things.
Besides, it would have raised howls of protests from fans that one driver change happened, but another didn’t, so fans of Senna would be upset that they could not play as him/against him. It was easiest to just leave the driver roster alone, and concentrate on more-important things. It probably would have required a whole lot of effort (that could have been better-spent elsewhere) for what would essentially be a cosmetic change.