Robbie has not crashed as much. He does more dangerous stunts because he can . Evel just did it by the seat of his pants. I have to say that Evel had more showman in him and less Rider skill. Robbie has more skill but less Showman. I saw Evel jump in 1975 at the Kings Island Park and it was really scary. Evel knew how to build up the crowd. When he went for the jump the whole crowd screamed as he went over the 14 greyhound buses.
If you're anywhere Butte Montana this weekend...
http://www.knieveldays.com/
The big reason Robbie is riskier than his father relates to the improved safety in the ramps and the equipment over the years. When Senior jumped over the fountains at Caesar's Palace, note the small ramp. At the time, ramp designs weren't as CAD-advanced as they are today, and Robbie is paying dividends for having better design technology behind the risk-taking jumps.
Also lighter motorcycles make a difference. His Honda CR500 is a class away from his dad's bikes. The CR500 is a purebred motorcross bike, designed for MXGP competition.