It's pretty simple.
Around the time of Max Payne 3, Rockstar changed their development model to have all their studios work on one game instead of having each of their studios make separate games.
On top of this, going into this generation, we switched from a standard development cycle of 2-3 years to a standard development cycle of 3-4 years.
Given all of Rockstar's staff works on one game, and that they're usually at least one year longer (though frequently two) with development cycles than everyone else, you would generally expect a new game out of them every 4-6 years.
You will probably only get one new game from them this generation, at least as a non-crossgen title.