20mm prime lenses aren't overly popular anymore these days - most people prefer ultra-wide zoom lenses or prime lenses with an even more extreme focal length. However, consider me to be a relic. I think that 20mm is actually a sweet spot in this focal length class. It is extreme enough to create a WOW effect yet not over the top thus avoiding an overly distorted view.
Anyway, the Canon lineup is the most extensive around so it doesn't come as a surprise that they also provide a prime lens here although it has not been updated for ages. The Canon EF 20mm f/2.8 USM has been released back in 1992 so it's one of the oldest members of the EF family by now.