Why does Darcy say Jane is a bad match for Bingley?

Darcy admits that the Bennet family's low connections and impropriety convinced him that Jane was a poor match for his friend. However, he adds that his main reason for dissuading Bingley was that Jane did not seem to show much preference for Bingley. Her attitude was always somewhat aloof, albeit pleasant, which is ultimately why Bingley started to believe Darcy's claims.