In the comments, Greg speculated that the Cartesians rejected Newton because his theory was unfamiliar and, they thought, incorrect. But the actual situation is almost the exact opposite: Newton's theory seemed all too familiar to them: they thought it was a throwback to Scholasticism. Moliere famously lampooned scholastic philosophers in a scene where they "explain" opium's sleep-producing properties as due to its "dormitive principle." Well, that was the way Newton's theory looked to the Cartesians: he was "explaining" gravity by an "attractive principle" contained in matter. It was not that they thought Newton's theory was wrong: they didn't think it had any explanatory power. There were Cartesian theories that contained inverse square laws, but which, to them, provided explanations of gravity, in line with their mechanical philosophy.