The Supreme Court is more of a check than an active force - but it is a very powerful check. History has shown that if they act without caution, they can easily make things worse than they were before. (See, for instance, Dred Scott v. Sandford, one of the causes of the Civil War...)
They are willing to step on toes if they need to - lots of cases, recent and historic show that. But they prefer to avoid doing so unless they need to, because it can cause problems. If the other two parts of the government are working on an issue, it's generally better to let them work it out - there will be more voices heard, and it's easier to adjust and make changes.
Basically, they are respecting that the other parts exist for a purpose, and attempt to let them fulfill that purpose. The Supreme Court's job is to step in when the other two parts fail - and it's not clear that they have failed here yet.