[deaddodo]

I couldn't tell you without a specific example, but I would suspect distributing an edited ROM would be much easier than save state hacking.

In a general sense, I suspect the latter is more popular because it's not illegal to modify/distribute save state data; ROMs certainly can be.

[anthk]

They distribute delta binary patches on the form of IPS formats.

[deaddodo]

Which gives me even less reason for why they would distribute modified ephemeral data vs source data to alter game logic.

Obviously, if you want all 151 Pokemon captured, you would do that through a save state. But if you want increased chances of capturing Pidgey, it would certainly be easier to distribute a ROM (or delte, in this case) with those new ratios baked in.