"Jesus saith unto her, I - am He (John 4:26) -
Our Lord never spoke in such direct terms concerning Himself to His own
countrymen; nor 
even to His own disciples, till a little before His
death.
The reason given by Pearce is the following: The woman
being alone when Jesus said it, and being a Samaritan, He had no reason
to apprehend that the Samaritans, if they knew His claim, would disturb His ministry before the time of His suffering came; which seems to have
been the reason why He concealed it so long from His own countrymen."
Adam Clarke