Most (though not all) offroad motors lack some of the more intricate machining on the crank. Look at the crank on an RB S3 (cheap motor) next to a C6. A lot of very difficult machining went into that C6 crank that is missing on the S3.
A second reason, offroad is a more price driven market. Any halfway decent offroad motor is capable of winning big races. Onroad is a performance driven market. Build a faster motor and someone will buy it even at 1500.00 street price (though not too many perhaps).