Might be because of different utilization rates. Cars can be rented 7 days/week between biz (weekday) and consumer (weekend) travelers, and across all seasons of the year. I’m guessing bikes get rented on weekends (30% of days) during the summer and spring (50% of days).
Rough estimates of course, but only 15% utilization (30%*50%) would radically change daily rental prices. Doubt it’s the only reason, but wouldn’t be surprised if it’s factor