The finance team understands that games have a life cycle. They know it's extremely unlikely for a video game to continue to grow for 10+ years (even WoW has tapered off since its crazy success). Their goal is to maximize value during the time the game has the greatest popularity. Welcome to Un'Goro. I would be willing to bet that quests have been kicking around the design floor for awhile, but the team knew they had to be legendary, and it would create a very expensive barrier to entry, so they had to wait until a certain point in the game's life cycle to not repel people with this kind of a mechanic. Imagine if quests had been released during GvG or TGT, when everyone was complaining that the game was P2W. It would've been much much worse and likely turned many people away just as the game was really gaining steam. An expansion like Un'Goro signals to me that Blizzard's thinks Hearthstone has hit its peak, and they want to start really cashing in.

editing because i had another thought about analytics

Also, do not forget that Blizzard knows EXACTLY what you're doing with respect to anything you as a player do in a Blizzard game. They have a huge advantage in monetizing you over other gaming companies because they can track how players allot their time between different games. They likely have customer segmentation models that track people who are diehard players of one/two games, people who switch back and forth between all their games, casual players, hardcore WoW players, people who are willing to spend, people who will die never having spent a cent on a microtransaction etc. Nevermind the kind of data they can purchase from other gaming companies. Here is an example of a guy who was "specially selected to receive" a free copy of Overwatch because he spent a ton on microtransactions in another game. The marketing team knows exactly who's playing what game, what their demographic stats are, what their purchasing preferences and patterns are, and any number of other metrics they want. They tell the finance team what kinds of content would be well received by the playerbase of each game. I'm guessing that Hearthstone is getting close to the point where new content is starting to appeal less and less to casual players. This means that Hearthstone is well into the mature phase of its life cycle and you can expect one or two more expensive expansions before they start shifting resources towards other games and projects.

a final edit for my editorialization

I'm extremely happy with the content in Un'Goro. I've played this game since Naxx, spent well over $500USD on it, and have had 1000's of hours of enjoyment. I will echo Kripp in saying that the cards in Un'Goro are the most interesting and exciting cards ever printed. I am not one of the cynics who think Blizz needs to change and appeal to the whole playerbase. I'm just explaining the rationale for the expansion from a business perspective. As a Blizzard investor, I'm happy with the direction they're going as a company and am extremely excited for the next few expansions in Hearthstone and for Overwatch.