For me at least they have a huge appeal because of the fact that it's a pickup. Which for me is pretty much the best solution to a vehicle for overlanding. Realistically any setup with just a cab and a bed/flat deck is ideal for my needs. The smaller Unimogs have always appealed to me as well. As for the 45, they have the classic and very nice lines of a 40 series truck, simple, clean and easy to repair if there is body damage as most panels are flat steel. They are rare out here, which also adds to the appeal. They came with 1 ton full floating axles and in Australia they came with diesel engines as an option. They are iconic symbols of the Australian outback. Realistically any old school pickup has that appeal, the Willys were beautiful little pickups as well, same with the rare Land Rover pickups. The difference is that the Land Cruiser 45 was built better and tougher than those other trucks and for a fan of the Land Cruiser it's kind of become the holy grail of trucks to have. For me, it's one of the best looking pickups out there and it fits my needs.
Other guys go crazy over the 45LV wagons or the 55s. For me neither of those options are as appealing as the pickup. For that matter even the 75 would be a nice rig to drive but the cabs in those are tiny, and I'm not a tall/large guy but it's just too small for me. So if the 75 had a 6" longer cab I'd be all over that setup.