1) One reason people like Java is reliability. Therefore adding Lambdas must not break any existing code at all. This includes no breaking changes to any part of the standard library.
2) As we can't make substantial changes to existing code, the lambdas we add have to fit well with existing code bases, without adding too many nasty corner cases to lambdas.
3) Once added, you can't ever make any changes to lambdas. This is a one-time opportunity. So you better get it exactly right, because you can't go back.
You say it is "never urgent" to make existing Java users rewrite their code. It is not "never urgent", it is a fundamental pillar of Java that users will never have to rewrite their code, and that is one of the major reasons that Java is so popular with business. You can argue that Java shouldn't make that decision, and should end up with something like the long drawn-out slow change to Python 3, or the Perl 6, but it was a conscious choice, not just something the developers can't be bothered to do.
Under those kinds of restrictions, I would argue adding lambdas to any system would be difficult. There were similar difficulties adding them to C++, and I would say in many ways Java is getting it better than C++ (but that is largely due to the much higher level of complexity of the C++ language).