Lightweight flywheel options and selection criteria.
Because he can't do it. Other than that, there is no reason to run a steel flywheel with a SLR of 10:1. If you can tune you can even have a taller SLR and use an aluminum FW.
99% of the issue is tuning. I've been running aluminum flywheels on everything I can since the mid 1980's. I would never go back.