Why did Jesus stop reading the Isaiah verse at that point instead of finishing the verse?
Jesus stopped there because the remainder of the Isaiah verse had not yet been fulfilled. Only the portion He read was being fulfilled in the hearers' ears that day, so He could not finish it. He knew exactly where to stop, ending at the point where fulfilled Scripture gave way to future fulfillment.
Les explains that Christ could not have said, “This day is this Scripture fulfilled in your ears,” if He had read the whole verse. The reason is that the last part of that Isaiah verse has not yet been fulfilled, so Jesus knew exactly where to stop. Les connects this to a similar pattern in Acts: in the Acts passage, He could have stopped at the word Samaria, because that is where it did stop. The Jewish disciples never went to the ends of the earth with the Gospel of the Kingdom because the nation was rejecting it. The point is that Jesus stopped at the boundary of what was then fulfilled.