Stack Exchange is based on having a big and increasing community of people who are interested in new things, in asking new questions and in answering new questions.
Sometimes a new community out there will start a big effort to recruit followers for an area51 definition phase. But they can be close for many reasons, even though they might be doing well in terms of follower recruitment and question upvoting.
As an example: Recently, SE closed a Neuroscience proposal when it was 6 questions away from reaching commitment phase. This proposal had attracted a large proportion of followers new to stack exchange. Some of these people were leading experts interested in establishing a community in SE's backyard.
Does it make sense for SE to close the site before it can really evaluate how many people are there committed this new site, what is their level of expertise outside SE, what kind of take they will have on the subject.
What I'm arguing is that: even if the proposal is a partial duplicate of another area51 site, it is not convenient for SE to kill the proposal before the new users (new experts) establish themselves as frequent users of SE.
In this recent example, the users of Neuroscience were referred to another site, but most of them stopped their SE momentum.