Research, and contributions to knowledge, do not come in standard sizes. The unit of currency varies by discipline, and some research projects take a lifetime, while some discoveries are made quite suddenly.
PhDs, however, have a constraint: they are designed to finish within 3-4 years of full time work by a new researcher. It is not just a matter of a contribution to knowledge (albeit small) but whether the contribution to knowledge is enough to merit a PhD.
So in addition to the qualitative judgements about orginality, rigour, novelty there are some important quantitative issues:
Is the scope to wide or too narrow? Do we need to triangulate with some more fieldwork? Do we need a further caser study? etc etc.
These quantitive judgements by the supervisors and examiners are justified by appealing to the qualitative issue about whether the conclusions has been "established" on the basis of the evidence used, or require further work.
Each discipline has its traditions about whether negative results represent contributions to knowledge, what sample sizes are required, whether the conclusions based on one study can really apply to other scenarios etc.
And of course, all research requires further work.
But while PhD students may be entitled as a last resort to submit in defiance of advice from supervisors, it is not to be recommended. Although this is yet another area where there are no clear rules, there tends to be a strong consensus among academic examiners from any particular discipline. And they are likely to ask about the progress of the research: what were the initial objectiives, what advice was received, what were the opportunities and setbacks? Why, in the end, was this particular result set deemed worthy of submission?
And of course, has any external body, conference, journal been impressed?