The only reason Big Basin had any old growth Redwoods is that it is in a big basin and it was too much effort to get in there and haul the things out. Everywhere else in the area (like up along Skyline drive) was farmed to the ground. The redwoods regrowth from that is now 100+ years old and getting pretty big again.
You are correct that a lot of remaining old growth Redwoods were in places that made it difficult to harvest, but some were deliberately saved, including the ones near Big Basin Redwoods State Park headquarters. Also the ones at Henry Cowell Redwoods State Park were pretty harvestable.