By America, I mean everybody. Fans, athletes, coaches, and especially Letsrunners. I have lots of thoughts on this, so I'm going to keep spilling randomly.
1. Because the world hasn't really gotten that much faster in a long time.
800: In 1964, Peter Snell won the Olympic gold medal with a 1:45.1. Since then, third place has never been slower than 1:46 flat, and first place has never been faster than 1:42.58. Only once has the winner run faster than Alberto Juantorena's 1:43.50 from 1976.
1500: In 1960, Herb Elliot won the Olympic gold medal with a 3:35.6. Since then third place has never been slower than 3:40.69, and first place has never been faster than 3:32.07.
Yes, world records have improved, but not really all that much, especially in the 800. Coe is still the third fastest man in history with his time from 1981, and Cruz is fourth with his time from 1984.
2. Because racially/genetically/physiologically, it's the perfect middle.
Shorter stuff? Dominated by West Africans. Longer stuff? Dominated by East Africans. 800/1500/mile? Not dominated by anyone. You have West Africans reaching up that far and East Africans reaching down that far, but everyone else is (literally) in the running. White dudes can compete. Coe, Ovett, Cram. John Walker, Herb Elliot, and Peter Snell. More recently, Nick Willis, Yuriy Borzakovskiy, and Mehdi Baala. So can Latinos, like Juantorena and Cruz.
In America, instead of the all-West African sprinting team and the all-white (with the occasional East African transplant) distance team, the mid-distance crew is actually interesting. Khadevis Robinson, Nick Symmonds, Leo Manzano, Lopez Lomong, Andrew Wheating, Charles Jock, etc.
3. Because according to Goldilocks-the-would-be-fan, it's the perfect middle.
The short stuff is too short, the long stuff is too long, but the middle stuff is just right.
Only die-hards love the really long stuff, and I don't see that changing. There'll be like one interesting move that takes a few seconds in a 27 minute race. The sprints will always be sexy cool, but that only goes so far. You can't pack stadiums with people only interested in the races that last a few seconds. Mid-D races are short enough that they don't get boring, and the championship races are fascinating tactically.
4. YouTube
You don't have to know anything about running to understand and appreciate: the 2008 U.S. Olympic trials 800, Dave Wottle in 1972, Manzano from NCAAs, Alan Webb's high school mile, etc.
5k/10k/marathon on YouTube? Forget it. 800/1500/mile on YouTube? Definitely
5. Every kid in America runs the mile. A lot.
It makes sense to Americans. They always want to know your mile time, and they always think it's fast.