For most hackers, there's about a 10-year window between when they graduate and when they start a family that they are active. People tend to hack the games they played endlessly as kids. For many of the really active hackers these days, that's just not NES/SNES games anymore. By my best guess, that's somewhere between PS1/N64/Saturn and Gamecube/PS2/Xbox/Dreamcast. That's the era that we saw game development slow down, as games became bigger and more complex. I think that's what's happening with romhacking too.
I think this is on the money. And these systems have not aged well. It's something like the uncanny valley, where the razor sharp edges and harsh textures are too realistic to have the charm of a crude analogue, but too coarse to be enjoyable now.
Older games, having simpler resources, focused on gameplay. You can fire up Skydiver on the Atari 2600 and begin playing within 1 second of turning it on, as opposed to 3 splash screens, a title screen, an intro movie, and a loading screen. It was fun to play over 3 decades ago, and it's fun to play now. Towering Inferno, Missile Command, Mario, Zelda, Tetris... Games accessible to all ages and skill levels. As a curmudgeon generalizing broadly, there have been fewer revolutions in game design in the last 20 years, and games have mostly just gotten prettier, while becoming harder to hack.
Hacking for more modern systems takes deeper skills. A kid can understand how to draw a 16x16 bit tile, but if you want to change graphics after the mid '90s, you need a 3D modeling program and the skill to use it; you have to make a shader in Photoshop, with UV maps, bump maps, MIP maps... Then the assets have to be transmogrified and imported, etc.
Modding newer games takes more specialized tools. You can edit the map of a NES game with some tile swaps or hex edits. If you want to change something on a post-'90s console, pray the dev team releases its tools, as Valve did for Battlefront 2 in the mid 2000s.
... And pray that the console hasn't been rigorously designed to thwart hacking, like the Xbox 360.
In a nutshell, the games from the childhood of hackers now in their heyday have not aged well and are harder to hack.