Welcome to Paradox Plaza!
Strictly speaking, there are no mods at all on consoles. A mod is a change to the game by users. None of the big console manufacturers (Microsoft, Nintendo or Sony) allow mods, for two reasons:
- The consoles are sold at a loss (the price you pay is less than the price it costs to make them). The manufacturers make a profit because they receive some of the money when you buy console games. So the manufacturers want/need you to buy official games and DLC, not mods that users share for free (gratis, free as in beer). As well as making a rule, the manufacturers have technical systems to make it difficult/impossible for users to change games.
- Console players expect games to work every time. But if anybody can make a mod, then sometimes there will be a problem. If a mod fails on a console, all you can really do is turn it off and start again. It's not easy to find the problem and fix the problem ('debug') without a keyboard and a proper operating system.
So everything you can add on console must be added by Colossal Order (CO, the developers of
Cities:Skylines). In practice, CO have some of the most generous policies in the industry. As well as allowing console players to buy official DLC, CO also upload some mods made by PC players. But they do some basic quality checks first because of reason (2). And because it costs them money to check the mods, they don't (and couldn't) upload all of them, which satisfies reason 1.
I don't know for certain why CO mainly choose props, but I guess that these are the reasons:
- It's much easier to check that a prop works correctly than it is to check a big, complex mod like Traffic Manager: President Edition. You would need to test a big mod for dozens of hours to make sure that it works right. But if you can quickly check that a prop can be plopped and looks OK.
- Some mods would need to be re-written for console, because they expect the user to have a mouse and/or keyboard. There are also some technical differences between the way that PCs and consoles work, although this is not a big factor for Cities:Skylines as the game as designed with this in mind. But who would do this work? Because consoles are designed to stop users changing games, the modders couldn't do it. And CO can't do it either: the console versions are converted from PC by another company, Tantalus. So CO would need to pay Tantalus to do it.
- Big mods usually need to be re-written ('patched') after every update to the base game. They also need patches to work with one another. So adding a big mod to the console game multiplies the work CO and Tantalus must do for each update. CO is a small company with 20-30 staff and only a handful of programmers. Tantalus is a similar size, I think. They can't do a lot of work without being paid.
CO has tried to solve these problems with the Content Creator Packs, which means console players can pay the modders and CO for the extra checks. But their teams are just too small to do this for big mods.