On PC at least it was probably because of the requirements. It was pretty rough on PCs. Works better using Proton than it did with Windows Vista or 7, though you can compile dxvk or whatever for Windows too to make it better now. On consoles, you needed a 2 year old console rather than a 4 year old one so I suspect fewer people had them this early into the generation. Later on, you’d get GTA V on the same generation.
https://en.wikipedia.org/wiki/Grand_Theft_Auto#Sales
GTA4 sold less copies than San Andreas.
On PC at least it was probably because of the requirements. It was pretty rough on PCs. Works better using Proton than it did with Windows Vista or 7, though you can compile dxvk or whatever for Windows too to make it better now. On consoles, you needed a 2 year old console rather than a 4 year old one so I suspect fewer people had them this early into the generation. Later on, you’d get GTA V on the same generation.