Why do outdoor saunas have different heating systems than indoor saunas? For example, infrared indoor saunas versus outdoor saunas that have heaters with rocks.

Outdoor saunas have traditional electric heaters in them because infrared is not effective in overcoming cool ambient air temperatures. For example, if it is 50 degrees outside, the infrared heaters will struggle to get the air in the sauna warm enough to be comfortable, and assist in your sweat. Infrared is thermal energy, mostly heating what it bumps into. It really does not directly heat the air. Traditional sauna heaters heat the air, and can work a little longer to overcome cold (even sub-zero) outdoor temperatures.