This loss can't be considered that heartbreaking because of the level of expectation surrounding it compared to the other examples.
18-1 was so devastating because the Patriots were clearly a much better team (the best in the league by far) and they just had a really bad game at the worst time possible.
The 2010 Celtics were never really meant to get as far as they did, considering all their regular season issues, their 4th seed, their age, etc. They overcame all kinds of odds, and fell just short of winning it all. That's not really a heartbreaker.
Also, in both the aforementioned cases, the team had won a championship recently. Buckner probably stands out above any of the others because it perpetuated the Red Sox curse. You can't really be heartbroken as a fan if your team has won in the last couple of years.