I think it's because up until then, Madonna had always collaborated with producers who were less well known - she had a gift for picking very cool collaborators & coming up with original material.
By the time she worked with Pharrell & Timbaland, others had already done it so it led some to believe that she was losing her touch & becoming a follower rather than a leader. She gets criticized when she teams up with well known artists, some people think she is clinging on to other current, popular artists just to stay relevant etc.
Robin Thicke didn't get criticized because that song was just one of those extremely catchy songs that took over the charts - it came out of nowhere & nobody had any expectations.
