“Stanley only used stereo on 2001 and Eyes Wide Shut,” Vitali told me. “The others were all in mono.” When I asked why a noted innovator like Kubrick resisted mixing his films in stereo, Vitali explained, “We used to send people to all the key cities to check the projection, and what they found was that nearly all the sound systems in the theaters hadn’t been looked at for years or even decades. Often one or more channels weren’t working, so Stanley decided it was better to record it in mono and mix it meticulously so that it would sound correct even if some of the speakers weren’t working.”
Source: “Stanley Begged Us Never to Talk About Him”: On Making Barry Lyndon | Filmmaker Magazine