Because they were literally right next to him in terms of the Force on the Death Star, and once he knew Luke existed it became easier to track him. Hell Vader only barely senses Obi-Wan on the Death Star, and that may very well have been deliberate on Obi's part, knowing he'd have to sacrifice himself. It's worth noting that he only senses the Force in Luke when he's actively using it in the trench run as well.
And no, it is not where Vader came from, it's where Anakin Skywalker came from, and it's a pretty blatant plot point that Vader discards nearly everything that made him Anakin.
He literally brought his mom's corpse to the same home Luke grew up in.... ---
