As Chapter One makes clear, it is rather an uncommon piece of luck for Magwitch to come across Pip rather than anybody else. This is primarily because of the location where Pip lives. Note what Magwitch does after finding out that Pip lives with a blacksmith:
After darkly looking at his leg and at me several times, he came closer to my tombstone, took me by both arms, and tilted me back as far as he could hold me; so that his eyes looked most powerfully down into mine, and mine looked most helplessly up into his.
Magwitch is fortunate to have found a small, impressionable boy who is an orphan who lives in the same house as a blacksmith, and is therefore able to take a file and some food for him and bring them to him. The fact that Pip is a young boy and clearly a very sensitive one, as is shown by his orphaned status and his tears at the beginning of the novel, makes it easy for Magwitch to bully him into doing what he wants him to do and to ensure that he will not tell anybody else. These are the personal circumstances concerning Pip that are so convenient for Magwitch given his situation.
Pip lives in a village about a mile away from the fens, a marshy isolated area not too far from the sea. This is a perfect distance for him to bring food (which Magwitch calls "wittles") to the convict, being neither too close nor too far away. Pip's father and mother are buried in the graveyard in the marsh where Magwitch (though Pip at this point doesn't know his name) is hiding, giving Pip a plausible cover for trips to the area. Most conveniently of all, Pip lives with a blacksmith, which makes it possible for him to get Magwitch the tool he badly needs: a file. Magwitch uses the file to get the iron off his leg, which both hurts him and identifies him as a convict. It also helps that Pip is a kind, impressionable, and vulnerable boy, easy to intimidate and yet thoughtful enough to bring items like brandy for him.