I’m curious to know if it’s because the top six are really good or 7-15 really suck.
Going by a quick eyeball test of the "standings by date" for the 2010s, it certainly seems like the top of the East (Top 6) is having as strong as a season as they've had this decade.
https://www.basketball-reference.com/leagues/NBA_2011_standings_by_date_eastern_conference.html
In the interest of not doing work here's my extremely scientific Suck Index
15. Knicks: Haha oh goodness yes.
14. Cavs: Suck for sure.
13. Hawks: Shouldn't suck as much as they have but boy have they sucked and even if they play up to potential they'll just suck less.
12. Wizards: Suck but a feisty kind of suck, they can scare and even beat good teams like they did the Sixers.
11. Bulls: Suck but with some promise, the coach seems to be driving the suck more than the players.
10. Hornets: They suck worse than their record but have pulled out some improbable Ws so another feisty suck.
9. Pistons: They don't suck - any team with a positive point differential doesn't suck. They can definitely suck on any given night but overall they're aggressively mediocre.
8. Magic: Don't suck, another mediocre one with legit but dubious chance for improvement.
7. Nets: Don't suck, they have a slightly negative point differential but they're 3 games over .500 with some injury and schedule adversity. They're only 1.5 games behind the Pacers who definitely don't suck.
In conclusion, suckage is a land of contrasts. Thank you for your time.