Why do you think the residents wouldn't answer Thomas' questions when he arrived?

Answering Thomas' questions when he arrived would have required the residents to reveal secrets that he needs to learn for himself. The residents' answers wouldn't necessarily be the same as the answers Thomas will discover when he experiences the Maze on his own. The residents might also hesitate because they don't want to anger Alby and Newt, who run the Glade according to strict rules.