Football has little parity because there are no such thing as salary caps or a Draft, which leads to the Bigger Clubs having all the top players. All the top players want to play for the best clubs, and these clubs can pay the biggest wages. There is also the problem the bigger clubs poaching the smaller clubs top young players and signing their best players (transfers)
The Parity Difference between the NBA and Other Football league isnt as great as you think either.
League/Tournament Winners in last 20 years
NBA - 9 Teams
EPL - 5 Teams
Serie A - 8 Teams
La Liga - 7 Teams
Champions League/European Cup - 18 Teams
Keep in Mind the amount of teams in the league and the fact that the NBA is built to have parity and its embarrassing that the NBA has not had more variation in winners.
I don't get the easier to cause an upset, cause it a lower scoring game argument. That just means it harder for the weaker teams to score a goal to cause an upset.