It takes preparation, resources, a vehicle, travel to the trailhead, motivation, effort, and miles of walking to get into trails in a national forest. Potential victims on a trail aren’t carrying many valuables because they’re out hiking.
A city park is within walking distance, requires no vehicle to get to, can be visited on a whim, and potential victims are more likely carrying valuables (jewelry, electronics, nice handbags). Criminals can visit the park, commit a crime, and disappear back into the dense city on a whim, without planning or preparing.
Most crimes in this context are crimes of convenience. Hiking long trails is not convenient.