There are a number of reasons why the Splinter Cell series has fallen out of favor with Ubisoft and the gaming public at large.
The last couple of Splinter Cell games did not do particularly well either commercially or critically. The series seems to have peaked at Chaos Theory there has been a general lack of enthusiasm for the series by both the publisher and game buying public ever since.
Splinter Cell is in a genre (pure stealth) that is notorious for high development costs and low sales. Even if the series was not already struggling most publishers are weary about the genre. Square Enix got rid of IO Interactive and Hitman because of how much money it was costing them and I doubt that we will be seeing another Dishonored title any time soon.
Even at it’s peak Splinter Cell was never the big dog of its genre. It was at best in third place behind Metal Gear Solid and Hitman.
Ubisoft has also decided that they prefer publishing games that fit into their Ubisoft Game[1]. These games are easier for Ubisoft to make, add DLC to and turn into Live Services. Splinter Cell doesn’t fall into this category so along with games like Prince of Persia, Brothers in Arms and World in Conflict all have been ignored for games that do fit into their preferred design.
Splinter Cell’s original designer Clint Hocking left the company in 2008 with Chaos Theory being the last Splinter Cell game he worked on. There is only enough time money and people to make games and without Splinter Cell’s main creative lead at the company to fight for its future the series was always going to drop down the ladder in terms of priority.
So why has Splinter Cell disappeared into thin air? Splinter Cell is a series that has historically as successful as Ubisoft would like, in a genre that is notoriously not profitable, whose last few games have not done very well and it competing for resources against games that have none of those drawbacks.