My players want their characters to know more people but I fear this makes things too easy

My players, while playing a campaign in the city they born, they were a bit frustrated to not know many things about the city and the people. This was my mistake, I like more a sandbox approach and even I didn't knew what the city had, due to this, my players were unhappy. I want to make my new campaign better, but at the same time, I don't want my players knowing everyplace and everyone.

Is there any mechanic I can use to generate NPCs that my players know, and each could potentially have bad relations to balance the fact they now someone that could help them as my next campaign have mystery elements.

I don't know if it was clear enough the problem and what I need to solve. Any other solutions to this so my players are not frustrated are welcome.