The older audience plays them mostly because of the nostalgic feeling and the simple design they are used to, while the younger part values the machines that as the matter of fact define what the whole free Old slot machine games experience was (and still is) about. They are perhaps old, but will never be outdated moreover, there are a big number of gamblers of all ages who still play them on a regular daily basis.
Even the most renowned software companies still create the old-school games.