Its my personal experience that Sanctuary Hills may have a settlement problem. I made a second game after my first finished, with only the original 5 NPC's. A notion which baffled me when other, under-furnished comparatively settlements ended with settler numbers in the low twenties.
After Putting Food, Water, and Power all above 75 with a whopping 300 in defense Using God Mode of course for the sake of testing and a full play-through later, No one ever came to settle in my Sanctuary Hills. In fact, NPC's like the recovering Alcoholic, now Cola-fiend from Diamond City, that I directly sent there, would disappear after the first in game week of being an established shop runner or farmer. This particular problem happened in the original play-through as well.
Keep in mind, the original play-through began and ended in the first week (Beginning, at Hour one of game release) of the games release, and the Test play-through within the next 2-3 weeks after. So patches may or may not have occurred within that time frame that could of affected the test, but in my case, did not.
Additional Unnecessary to ask questions are answered with, Yes, I kept my Defense score 3x's higher than my resources combined.
Yes, I put a Satellite beacon in EVERY settlement I controlled, Including Sanctuary Hills. (Though not immediately on the original Play-through.)
Yes, the first play-through was almost entirely unmodded. (Save for texture replacers.) The following were not.
Yes, I updated whenever possible. GEForce Drivers included.
Best Answer
Because there is noone.
First: It doesn't matter how high your food, water and power is, for settlers to come to your settlement. It impacts only the happiness of your settlement.
I think the problem with Sanctuary is, that there really just aren't any humans around. The recruiting beacon has a limited range. Very limited. Test it out by going away from one in a straight line. you will quite soon recognise the "Recruiting signal lost" notice on the top-left of your screen. Especially if you set up a beacon in red rocket, the settlers will propably dragged there first. So, unless a patch will add some wandering wastelanders in the north-east corner of the map, Sanctuary will propably stay quite a silent little settlement (Except you send settlers from satelite settlements of course).