I've came across this reason for rejection of a proposal:
This proposal covers the same ground as another proposal; it would tend to drain audience from another Stack Exchange site.
Why is the latter part of the reason something negative. First of all, the audience will not get lost, it just migrates somewhere else within the SE universe. And secondly, in my opinion that's actually a benefit when a specific audience is getting gathered instead of getting ignored (or at least not getting enough attention) on other, more broad platforms.