The Enterprise is the Flagship of Starfleet and one of it's main jobs is to interact with Ambassadors of other planets for diplomatic reasons. Voyager and it's crew before or after the Maquis was not a ship for diplomatic interaction. What you're seeing is the difference in crew are professionals trained in diplomatic interaction with government officials vs. one that isn't. They're stuffy cause they're taking their jobs seriously and professional. Most of Voyager's crew are now people that could barely handle being discipline in their personal lives much less professionally.