Because he keepeth not the sabbath (hoti to sabbaton ou tērei). This is reason (causal hoti) enough. He violates our rules about the Sabbath and therefore is a Sabbath-breaker as charged when here before (Joh 5:10, Joh 5:16, Joh 5:18). Hence he is not “from God” (para theou). So some.
How can a man that is a sinner do such signs? (Pōs dunatai anthrōpos hamartōlos toiauta sēmeia poiein). This was the argument of Nicodemus, himself a Pharisee and one of the Sanhedrin, long ago (Joh 3:2). It was a conundrum for the Pharisees. No wonder there was “a division” (schisma, schism, split, from schizō) as in Joh 7:43; Joh 10:19.