The way it is these days is that your factory riders are contracted to do a series, whether it be Moto GP, WSB, BSB, AMA or whatever other, and the teams do not allow their prize investments to risk riding at the TT or any other race for that matter that might affect their results in their contracted series the only exception to the rule seems to be the Suzuka 8 hour which the japanese treat as being very important. I feel this is the main reason the days of top factory short circuit riders at the TT are over, they employ specialists to do the roads.