It's a pretty obvious one and it's something that happens to just about every company at some point or another once their success hits another level. Is it because the developers don't have the same passion they used to? Nope, that's not it at all. Is it because the resources of the company dwindle or go down? Nope, that's not it either.
What determines when a company's game starts slacking in quality?
The answer is simple really... when the game stops being made by gamers and being made by suites.
Look at Blizzard Entertainment for example. It was no secret that Mike Morhaime and the developers were die hard nerds and loved what they did. Did they make money for what they did? Well of course, they had to make money to support developing great video games and with each release, you can tell that their was passion in the content they released. Fast forward to July 9th, 2008 and that's when (in my opinion) Blizzard made the worst possible mistake you could ever make... merging with a company such as Activision.
If you know anything about Activision and the CEO behind it, you know that he's a money driven greedy dick. If the world was ran by him, games wouldn't have a cost but it would be individual based on the amount of money owned by the person buying the game. It would more than likely cost 75% of all your money. The guy has way more money than he'll ever know what to do with, so will his sons and grandsons down the family line but he's still constantly thinking of ways to nickel and dime people. Anyways, enough about the merger.
The problem is once a company goes public, it no longer answers to the public. They still have an obligation to please the public but it answers to the investors. Without investors, the company would essentially tank and lose a good majority of their money. What do investors care about? Investors don't look at the quality of the video games or if the customer base is happy, they could care less, they look at the assets of the company and how much money it'll make them.
Take a look at Diablo 3 for example... when Jay Wilson was presenting some videos and older videos of the game, it was simply amazing. Everyone was hooked and it seems like the resources were being put into the right place. However, to cut down on resources and development cost, they pulled a ton of the features, reverted a lot of beautiful interface changes back to basic and drawn out interfaces and allocated the team elsewhere. The result?
They sold over 12 million copies of the most anticipated game of all time simply because of the name... and where do those numbers stand today? How many of those people angrily fought with their credit card company to get a refund, uninstalled the game or just straight up refuse to play it? I don't know the exact number behind the D3 development team, but it doesn't take a developer to know they're moving at a snails pace.
It's not because companies lose passion for what they do, it's because instead of answering to the customers or the loyal fanbase, they instead start making their games based on how much money it'll make them.
Look at GGG (Path of Exile). The game's not bad, it's developed by a small handful of developers and it's FREE. However, wait until they either A: Go bankrupt or B: Get bought out by another company. They'll take the path that every beloved OG company has.
WWE, Blizzard Entertainment, those are just a couple of my two most loved companies from back in the day that sold out and stopped making products for the fans. It's a shame really.. to see just how corporate this world is anymore and to see how everything really is about the money.. even down to the nickels and the dimes.