The reason is likely because, although the other guys aren't saying the car can be used for pleasure drives and other uses, Hagerty has it in writing. Just be wary of what they say because unless you have it in writing they aren't going to stand by it.
I had American Modern, which is through GEICO (IIRC), and they had it in writing that I could use it for pleasure drives. I only had about $30K in coverage a the time because the cars weren't getting crazy expensive and I paid about $475.00 per year.