Seems like they didn't account for players playing to win. With the amount of money on the line that epic threw at this, someone should have realized that players would go for winning strategies over fun strategies or entertaining strategies. Hell, the only reason that teams in TF2 don't park the bus after one round win is because there's not enough money in it to do the most boring, awful strategies to win. For a grand prize of more than I make in a year you'd bet your ass I'm playing heavy on spire for an hour with an engie and a pyro to back me up.
Any potential esports future for Fortnite would probably need to see gameplay significantly different than what the casual game is. Probably a much smaller pool of players to start with, maybe a smaller map or constant shrinking of the circle, definitely something to put time pressure on players to limit the effectiveness of waiting. Maybe some kind of reward for aggression, since, all else being equal, the optimal strategy is to take as few fights as possible.
As it stands, epic tried to hammer a square peg into a round hole by pushing Fortnite as an esport. It's designed from the ground up as a casual, drop-in drop-out game, and could excel on that basis alone. Successful sports require skill-based play, an enjoyable or accessible viewing experience, and gameplay that's relatively consistent from the lowest levels up to the highest. Fortnite has, arguably, one of those things, and without some drastic changes, will probably either fail as an esport, or "succeed" as long as epic is willing to dump money into it to keep the hype machine going.
TF2 has largely failed as an esport for the same reasons