Because it's what people expect from a sequel, if you want to experiment, you make a spin-off game, a DLC, or a new series.
Really? Says who? I get that traditionally most sequels follow the formula you are suggesting SR should have, but so what? Look, I would be more inclined to agree with you if Volition had handled SRTT, SRIV, and GooH the way that Rockstar/Take2 handled the Defective Edition of the GTA Trilogy, disseminating almost zero information and then an intelligence-insulting review embargo to make sure that people had no clue what they were buying until they actually started playing the game(s) after paying full price.
But Volition didn't do that, did they? They were open and up front about all those SR sequels right from go. Anyone who didn't like those SR games because of the direction Volition went truly has nobody to blame but themselves, because it was so obvious the vibe, feel, and gameplay that those games were going to have long before they launched. Even pre-orders could have been refunded from any retailer if the players who had serious issues with SRTT, GooH, and SRIV had just taken the time to see what those games were going to be based on all the pre-release information.
So, no, Volition is under no obligation to keep a franchise going in a particular direction, just because a bunch of people were too dumb to actually look at all the pre-release videos, preview information, and story leaks, and just bought it anyway.
By the way, I believe most people actually liked those games, as evidenced by the sales and preorders for SRIV and even the sales of GooH, after so many people played both the previous full games. Not to mention all the excellent reviews.
And in case you want to chuck in the old cliche that "reviews don't matter, dammit!" I would just like to point you to all of the almost universally horrible reviews of SR:2022. See? The system works, after all! lol The sales of that game were awful, and so were all the reviews, and for good reason.
Oh, and on the same topic of knowing what you're getting into before buying? I didn't purchase SR:2022. I have very little interest in that game until it ends up on Steam, and is given a huge discount. Why?
Pay attention now, because here's the important part:
Because I paid attention to all of the pre-release information for SR:2022, and decided it looked like crap, and so I wanted to see of the reviews told a different story, which clearly they didn't. So I passed. Simple. Problem solved. Oh, and also in Volition's defense, no matter how shitty SR: 2022 ended up being, at least they were honest about what they were creating right from the beginning. They didn't promise SR like the old days and deliver some woke-ass, pussified, duplicitous version of the game. From everything I have seen, SR: 2022 is pretty much everything Volition said it would be, at least regarding content.
The GARBAGE and buggy state they released it in is another story, and they should be hated on for that. Releasing a glitched out, poorly running mess of a game is not the same thing as I am defending in this post.