Luke was being protected by the Emperor in the OT.
All this time we thought the Lars Family were just good samaritans. (Well, Aunt Beru was nice...) It turns out that Owen was under orders from Palpatine to keep Luke out of the way until he was ready to succeed his father! But that's not all.
Owen comes by this treachery honestly; he gets it from his dad! Kliegg was ALSO under orders to buy "The Chosen One's" mommy dearest out of slavery and help set up the events that led to her demise. It turns out that Palpy orchestrated the whole thing just to get Ani to start down the path to the dark side.
But once Ani became "damaged goods," Palpy had to start a new search. Along comes Luke. Clearly both Vader and Palpatine knew of "the son of Skywalker" enough to perceive him as a threat. Now we know how.
Here we were wondering, "Why wasn't Luke protected by Kenobi?" Because he was spying
for Yoda, explaining the line, "This one, a long time have I watched."
For the record, Beru was oblivious. She was fed the same line of bull Owen gave Luke about his dad being a freighter captain. When she said, "Luke's just not a farmer, Owen. He has too much of his father in him.," Owen replied knowingly
, "That's what I'm afraid of." Now we know why.