Why the change? Blizzard posted on their HearthStone blog:
At their core, Adventures paint colorful and thematic stories about the Hearthstone universe. Imagine being able to learn about the history and lore behind each of our sets inside the game in a similar way! Defining who the Jade Lotus are through a series of battles between the Mean Streets of Gadgetzan families would have been a great way to supplement the overall Gadgetzan story arc. That concept is something we would like to do more of in future sets: Take the storytelling and cool missions of Adventures, and combine them with the card pools of expansions.