Because other EU countries were in trouble, too. If Greece defaulted back then, investors would have lost their bounds and countries like Spain would have had big problems to sell their bounds.
It is much harder to consolidate your finances if you have to pay higher interests than already solid countries.
Before the Euro countries had a mechanism to tackle this: They could just print more of their own money. Italy was an example for that. They just weakened the Lira while Germany did not weaken its Deutsche Mark, so it was easier for Italy to export and harder to import goods.
The US entities that default do not have this problem, because there is always the state and federal governments that can step in after the default and help finance them. The Euro group did not had any entity that could help before the crisis. Now there are two mechanism ESM [0] and EFSM [1], but these two are essentially just fonds + some rules and can help only to a certain extent. They are too small to save a country like Italy or Spain if it defaults totally, let alone several of them. There is no entity above like the US federal government except the ECB itself, which does not have the mandate to do stuff like that (even if they do it anyway [2]).
Another reason is of course that state bonds are considered safe, so French and German insurance companies and the like invested in them, which would have needed to be bailed out later on anyway.
I think if Spain, Ireland etc. would not have saved their banks back then, it would have been safer to let Greece default. But we do not know what would have happened instead and it is always easier to say would should have been done afterwards.
[0]: https://en.wikipedia.org/wiki/European_Stability_Mechanism
[1]: https://en.wikipedia.org/wiki/European_Financial_Stabilisati...
[2]: http://www.forbes.com/sites/jonhartley/2014/10/14/legality-o...