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<p>I asked a professor about this, and he said that admissions into the PhD program for MS holders is harder because if they fail the qualifying exams then they walk away empty handed. At least if you are a BS holder and fail the qualifying exam, you can leave with an MS. That’s the only reason he gave me.</p>
<p>I also think that admissions requirements are more stringent for MS holders. They have to show that they can handle the coursework, and probably expected to have some publications in addition to a thesis. These are just my speculations.</p>