I think to some extent it has to do with more people knowing at least a little assembly. Older hacks were more asset-focused (text, graphics, maps, etc). I feel like there are more behavior-focused hacks now that involve modifying existing routines or creating new ones. Doesn't explain everything you mentioned, but I think it at least explains part of it.
Back in the Dark Ages, knowledge of assembly was extremely rare. This was due to lack of documentation more than anything, in combination with a lack of good debuggers/IDEs to work with.
Emulators like NESticle allowed non-programmers to create their own graphics edits, and utilities like Hexposure and Thingy allowed easy text editing. While a handful of good hacks were made at this time, most were poor (And typically immature as hell) graphics edits and/or spoofs.
A bit later the first level editing utilites started to show up (Mario Improvement being the first one I can recall), so that started becoming a prolific feature in hacks.
I remember things started getting better for assembly level hacking when Parasyte created a debugger and bolted it on to FCE Ultra (FCEUD), which was then further improved by bbitmaster (FCEUXD): For the first time good, powerful debugging capabilities were available and hacks that would have been fantasies just a few years earlier were suddenly pretty easy to pull off if you read a few documents.
It's easy to take for granted how relatively easy it is to find utilities and information now unless you were there in the 90s/early 2000s. The tools available now vs when the old hacks were done is like comparing an atlatl to an assault rifle in terms of technical sophistication and power.
As far as hacks that create original games and such: I can't stress how fucking difficult and time consuming
it is to do a "total conversion" type of hack: Those have never been common, at least ones that were ever completed.
(As an aside, Vengeance on Hell is what originally inspired me to do conversion type hacks: The quality wasn't very good, but it showed me that much more was possible with ROM hacking than the naked Mario and racist as hell hacks that were common at the time).