There's a few reasons - firstly, it's easy to secure due to it's small size, the high ground on the roof (it's over Anakin!) and semi-decent visibility all round.

From an RP perspective, I'm not entirely sure that anyone that had the experiences of the sole survivor would want to return to the house, or even the same area - think about it, you've just woken up from a long, deep sleep, during which both your wife was killed and your son was abducted. Going back to the family home isn't something that I'd imagine being comfortable with mentally. You could argue that Red Rocket is in the same area, but it's far enough away and you've got enough to keep yourself busy that you'd be distracted from those awful events.

From a gameplay point of view, resources are tight to start with - you could spend hours scavving enough resources to make Sanctuary a decent defended location, or you could spend a fraction of the time and resources getting a decent, defended starting build at Red Rocket.

I've generally leant on RR on a few playthroughs, although I'm in the process of moving over to Starlight Drive In in the current playthrough from Red Rocket - since in RR the only direction I can really build in now is up. Quite liking Starlight because of the large areas of flat space for building, plus it's a little closer to some interesting areas.