It's going to be difficult to really choose a "best answer" here since to some extent the factors involved, and especially which ones are "most important" is going to be a matter of opinion. And this is because economics is not a settled science in the way that physics or chemistry are, such that there are "correct answers."
That said, while it has already been noted that many people DO consider the US debt to be a big problem, and relative population and relative GDP do certainly matter. The one I'd emphasize above all has mostly just been mentioned as an "also" reason.
And that is ... the denomination of the debt. The US can print dollars. Greece cannot print euro. You don't have to think this through very hard to see that this means the US can ALWAYS pay its debt in dollars, while Greece cannot say the same in euro.
Now, if the US prints dollars to pay its debt, that can cause inflation, but the cost of inflation is borne by everyone while the cost of default is borne by only the bondholder. This is not only a difference, but a vast difference--one which is easily large enough to change the result.
In the case of Greece, and countries like Greece, default on sovereign debt causes the banking system to fail, which is an existential problem for a modern economy. In the case of the US, and countries like the US, high inflation erodes wealth and creates various pernicious incentives, but it (almost) never causes the banking system to fail, although in very extreme examples it has been seen to do so occasionally, for example you could look at the Weimar Republic between WWI and WWII.
But if you did look at Weimar you would see what an extreme case it was, Germany having lost WWI and been saddled with huge reparations by the Treaty of Versailles, and the US is nowhere near that situation.
Bottom line, in Greece you get a collapsed banking system. In the US you get inflation. Many regard inflation as a problem, and rightly I think, but it's not as bad a problem as a collapsed banking system.
In my view, that is the big "why." It is the one which, so to speak, is a difference in kind rather than a difference in scale.