The problem with Superman as a video game hero is pretty much because he is Superman. The character has flight, super speed, super strength, eye lasers, etc. etc. etc. He can do so much. So how do you implement this into a game? Do you rip away his powers (Superman forgot how to fly..)? Do you give ever enemy Kryptonite (A material that is both radioactive and super rare to find)? Batman games can be executed properly as he is human and though he has his various tech, it isn't something drilled in to his DNA and is a literal part of him. Do we take the game and place it on another world perhaps, scrapping a game that is set in Metropolis? Possibly. Ultimately there are a lot of factors that past developers just don't want to deal with. Maybe this will change in the future, but with the past Black Mark on the title when it comes to gaming, I am uncertain if we will see another outside of a video game movie.
Now, first off I am tossing on the record, I enjoy Assassin's Creed. However...
...Now with Assassin's Creed, I won't lie that I like the original premise. You were the main character, an individual named Desmond, who descended from a lineage of assassin's and you are being brain probed in order for the enemy faction who is hundreds of years old to find what could be a super weapon. The game was supposedly be a trilogy, with it's release date originally the same day as Bioware's game "Mass Effect". Assassin's Creed one was semi enjoyable, but the main character was unapproachable and hard for many players to relate to. Not everyone was raised to be a Master Assassin with limited people skills. From their, they progressed on, and instead of ending it as a trilogy, they drew it out longer. There is a reason that after Assassin's Creed 2, we saw Revelations and Brotherhood instead of 3.
My problem with Assassin's Creed is not so much it is Assassin's Creed, but that since it was so successful, it has become a basis for sandbox games. Look at Watchdogs. You might not be scaling buildings, but the concept is very similar. Area is broken into regions with a centralized spot needed to be "activated" in some way in order to unlock sorts of sidequests in the area. Rinse and repeat.
Now... there were some things that I did enjoy with Assassin's Creed. For Black Flag (AC4, they incorporated a huge world, but more importantly they allowed you to transition flawlessly between land and taking the helm of a ship, taking to the sea where you could adventure, follow plot, but more importantly, attack other vessels, looting and pillaging. AC4 had a story line... however, the amount of time I spent on it was minimal compared to the amount of time I decided to turn the game into "Yaargh! I'm a pirate!" simulator.
With the upcoming Assassin's Creed Unity (Numbers are a silly thing), the thing I am seeing I am liking is the concept of a Co-op campaign, meaning that instead of merely being the hero (Thieves and Assassin's are heros? I tend to have them as my preferred character class, so yes.
) Some games have attempted this before outside of an MMO and it is something that we will be seeing more of. Some games did it successfully with the ties being a bit loose (Borderlands 2) and other's did it with a tighter knit bond, which had mixed results (Army of Two).
As for why this game does well, the answer is simple. Many games that try something somewhat radical or different are typically not accepted into the majority. This is why some games become classified as cult classics (Nier, Mirror's Edge, Deadly Promonition, etc.) As much as I would love to see the games cycle out, Assassin's Creed has become a name brand, like Call of Duty and Grand Theft Auto. People will play it because of the name.