I believe zebo has the nail on the head. Reality TV is cheaper to make generally. The shows that feature so called celebrities must cost less unless they do it free, which is highly unlikely.
Chasing coppers doing their job, emergency rescue services, bailiffs, heir hunters, cooks, dancing, ice skating, singing, house hunters or builders etc, etc, etc probably come cheaper but once you've seen one they quickly become repetitive and boring.
I record a lot of stuff which looks interesting. This has the added benefit of fast forwarding through any annoying adverts. If the programme turns out to be rubbish the delete button quickly gets rid of it.
The one exception, if it classes as a reality show, I thought Sunday's Top Gear was hilarious. The team were tasked with the job of building a bridge over the River Kwai in Thailand. First off they had to drive three clapped out old trucks 1200 miles from Rangoon across Burma(they refused to call it Myanmar) into Thailand to the river. Next weeks episode is the bridge building.