Question

why doctor strange didn't use time stone in infinity war

Answer 1

hey mate
time stone was gone because doctor strange had to save tony stark from thanos

Answer 2

In universe, because Dr. Strange saw all 14+ million possible futures and there was only one that led to victory for the Avengers and co--the one he took. Presumably taking any other path, including going back to warn the others, would lead to one of the futures wherein Thanos "won" forever. He says as much to Tony near the end: "It was the only way."

Out of universe, because a movie where Dr. Strange fixed all the problems via time travel would be pretty boring and wouldn't feature the rest of the cast in any meaningful way.