Probably because the population of black holes/neutron stars/brown dwarfs needed to explain the behaviour was thousands of time higher the upper end of expected interval determined from stellar evolution.
Which WOULD have been a valid point to raise and is one of the reasons I don't believe that particular theory either. That's not what worries me.
What worries me is that a surprising number of scientists accept that reasoning as a valid refutation without actually running the numbers to be sure. And the 2002 paper did not even appeal to numbers, but to a variation on the Argument from Incredulity: "If there were a lot of brown dwarfs in the galaxy, why haven't we seen them?"
The reason this worries me is that it demonstrates -- at least among cosmologists -- a willingness to discard possible explanations based less on data and more on preconceptions. When new data is introduced that would otherwise force them to reexamine those preconceptions, the tendency is to try to integrate that new data into the existing model rather than examine the model itself. The result, IMHO, is coming out to be an overly sophisticated model with more exceptions than rules and where the overwhelming majority of actors IN the model have not actually been proven to exist.
Lastly, the VERY worrisome thing about is is that cosmology is inextricably tied up with very expensive and very risk-averse space science projects, telescopes, satellites, laboratories and specialized equipment. With tens of billions of dollars on the line, there's pressure on scientists to achieve useable results, or if they don't, to justify the lack of results, often by calling for even more sophisticated instruments. No scientist -- no matter how honest or professional -- is going to go before the committee that funded a 350 million dollar satellite and tell them "All 50 tests were negative: no sign of dark matter." His career and the careers of his colleagues depends on his ability to spin that as "Our instruments detected [insert technobabble here] which could be consistent with some types of exotic particle processes. It's unclear whether this is suggestive of dark matter or some other process, but with more data the picture might become more clear." I've been in that situation before myself: Your can't say "Fail" so instead you produce a complicated "Did not entirely succeed."
That's bunk. Astrophysicists wold be thrilled to find a hole in GR.
There ARE several holes in GR. A dizzying number of them, in fact. I'm not speaking hypothetically here, I've been told by astrophysicists specifically that drawing any attention to the problems of General Relativity is an implicitly bad career move in some labs and especially in universities.