why it is so difficult for judges

During MBA, I took this course Business, Government and Law. There we get to know that Constitution is finally a living document. One need to interpret constitution in the light of incidents that happens today. The boundaries of law henceforth shall change and any robust constitution needs to have the flexibility to allow that change but in the process protecting the core values.

Another aspect is creation of institutions which can keep checks on each other and doesn't grant unilateral power to anyone for it to be finally abused.

The third part is what Justice Souter states here.
"A choice may have to be made, not because language is vague, but because the Constitution embodies the desire of the American people, like most people, to have things both ways. We want order and security, and we want liberty. And we want not only liberty but equality as well. These paired desires of ours can clash, and when they do a court is forced to choose between them, between one constitutional good and another one. The court has to decide which of our approved desires has the better claim, right here, right now, and a court has to do more than read fairly when it makes this kind of choice."

One can appreciate the difficulty that judges face when they have to decide between good and good. The choices are more difficult and one has to keep decisions in perspective of future, how will those decisions be interpreted in future.

Constitution will always have general guiding principles. Having set of rules for every current and future circumstances is impossible to achieve.