As an experimental physicist, I want to point out that negative results do get published -- for example, much of the experimental work on gravity amounts to the showing of no measurable difference from theoretical expectation, to a high degree of accuracy [1] -- but writing the papers for them is much harder than writing up positive results. The reason is simple: you have to convincingly show that your failure to show a result is not just due to your mistakes.

Here is an analogy to a trivial situation. Which is more convincing?

Positive result: By following the steps in the documentation, I installed MS Word on my computer. Therefore, MS Word can be installed on my computer.

Negative result: I followed the steps in the documentation, but MS Word still doesn't work on my computer. Therefore, MS Word cannot be installed on my computer.

If you're like me, you barely pause after reading the positive result, but the negative one brings to mind piles of questions: did you have the right version for your OS? Do you have enough disk space? Does your computer work properly in other respects? A really tricky problem could take ages to figure out. At some point it's probably going to seem wiser just to abandon the problem, and get a different computer or a different program.

So, as a scientist, when you're faced with a negative result, you know you have a battle ahead of you. If you don't think anyone will care very much about your result, moving on to the next project may seem to be the right choice. It's a tough situation, and I sympathize with anyone facing it. And is it the right choice for science as a whole? Sometimes it isn't, but sometimes it is.

[1] For one such example, check out the Eotvos experiment and its many descendants. http://en.wikipedia.org/wiki/Eötvös_experiment