Let's go with official words forof Joe Russo himself:
Speaking to IGN, Infinity War's co-director, Joe Russo, explained why Thanos is dressed down for the battle ahead. "He has a more casual approach in this film and it's very philosophical for him. It's part of his character definition. It's a spiritual journey for him to collect the [Infinity] Stones and one with which he does not need armor. Once he starts acquiring the Stones he doesn't require armor the same way he did when he was a warlord. It's a very symbolic costume," he said.