The Toronto Maple Leafs are one of the most storied franchises in the NHL, being part of the Original Six teams that comprised the earliest days of the league. Despite their long tenure, the Maple Leafs have struggled with success in terms of championships, having not won a Stanley Cup since 1967.
It's worth noting that Pacioretty is known for wearing #67 on his previous teams, so his choice isn't meant to provoke or disrespect the Maple Leafs at all. He's worn this number since the beginning of his career -- perhaps why he feels it will bring the team better luck.