In an era where games practically play themselves, it should be considered virtuous to make a game with rewarding difficulty. Apparenlty, it just means fewer sales.
When speaking to Eurogamer, Ubisoft Toronto Managing Director Jade Raymond emphasized that Splinter Cell would be much more popular if it didn't stick to its formula. She said:
One of the things that held it back is despite all of the changes that have happened over the years, it’s still one of the more complex and difficult games to play.
She added:
By default there aren't many games where that's the phase. Most games you can walk in and you start shooting right away, or you just walk in and you improvise as you go along. But Splinter Cell still really is a thinking game. It's really about being intelligent and taking that time in the first phase to plan out how you're going to do things, and understanding the elements, and even planning your gadgets and your load-out and being smart about it.
The issue has more to do with the stealth action genre than Splinter Cell. Series belonging to the genre, such as Splinter Cell, Metal Gear Solid, and Hitman, have to be more methodical to find success. It's easier to convey entertainment value to a player through gunplay and explosions. The same can't be said about sneaking through a building undetected.
Despite this, Splinter Cell and games of its kind continue to sell millions of copies. There's certainly a market out there that loves the thrill of a good stealh game, and while it might not be as large a demographic as the one who plays Halo and Call of Duty, it's substantial enough to keep the genre alive.