Most developed nations have extradition treaties with the US - and Assange has a good chance of independently pissing them off in the future if he leaks info on them.

Wikileaks has already leaked private bank memos, which the Swiss in particular don't take kindly to.

Ecuador is no fan of the US, and it tends not to be a very interesting nation from a leaking perspective. It'd be a better choice than most developed nations.