Young pups need to eat more frequently, so 3-4 times a day is recommended. As they age, their digestive system can process more at one time so going to twice, or even once, a day is what most people do.
As for why they go to 1-2 times a day, for most it simply fits their schedule better. Especially for those who work outside the home so feeding lunch isn't possible.
Fewer meals also makes it easier to plan training times around when the dog is hungry (and thus more willing to engage and work for food rewards). Bloat can also be a concern, as it is generally recommended not to let a dog exercise hard with a full stomach, and if the stomach is always somewhat full that could cut down on available time to exercise. Though with multiple small meals there probably isn't enough in the stomach at any one time to make this a significant concern.