My hunch was wrong. I checked the conjunctions in Matthew 5, and interestingly, Jesus used the conjunction de, not the stronger adversative alla…
Matthew 5:22 But I tell you, ἐγὼ δὲ λέγω ὑμῖν
Matthew 5:28 But I tell you, ἐγὼ δὲ λέγω ὑμῖν
Matthew 5:32 But I tell you, ἐγὼ δὲ λέγω ὑμῖν
Matthew 5:34 But I tell you, ἐγὼ δὲ λέγω ὑμῖν
Matthew 5:39 But I tell you, ἐγὼ δὲ λέγω ὑμῖν
Matthew 5:44 But I tell you, ἐγὼ δὲ λέγω ὑμῖν
So why didn’t Jesus use the stronger alla to show the sharp contrast between the Jewish traditions and His own teaching? Probably because in all six cases, Jesus had already created contrast by starting with the pronoun ego, which is emphatic. Lit., He says, “But I Myself say to you.”
Leon Morris writes, “[Jesus] uses the emphatic ἐγώ. France comments, “This is not a new contribution to exegetical debate, but a definitive declaration of the will of God. It demands (and receives, 7:28–29) the response, ‘Who is this?’ Thus this passage contributes another aspect to the presentation of Jesus as the Messiah which is Matthew’s overriding purpose.”
Using the stronger adversative alla may have actually stolen emphasis away from the pronoun, where Jesus wanted all the shock and emphasis to land. So Jesus instead used a weaker conjunction and put all the emphasis squarely on the pronoun ego. A new authority had arrived in town. His name was Jesus, and He was singlehandedly overturning centuries of oral tradition with a single sermon.
Stephen