Well, What if they port Tmodloader?
It would not be allowed by Microsoft, Sony, Nintendo, and Apple on their walled garden platforms.
This is because tModLoader lets you basically directly rewrite the game's code. And that's a potential security issue, because a mod's code then gets the same level of access to the hardware as the game's code... so just like the various console companies want to control what gets put on their systems so game developers have to send them their code for approval before publishing it, then
mod developers would also have to do the same thing and that's just not a reasonable prospect.
Here's an example of mod to show what I mean:
UltraSonic. A cool little mod that lets you have an in-game jukebox and replace the game's soundtrack dynamically... by accessing the Internet, downloading files, and storing them. Now that mod itself is perfectly fine, but think a bit about what, theoretically, a malicious person could put in a mod to harm the user... The simple fact that the technical possibility is there is enough to doom the idea.
When mods are allowed on consoles, it's because they have to follow a constrained modding interface that only lets them do operations that have no risk associated with them -- no access to the filesystem, or to the network. Just the game functions that the game developers made an explicit hook for. A good example of that is Skyrim. On the Xbox, mods can only use the game's native "Papyrus" scripting language. On PlayStation, they can't even use custom scripts like on Xbox, instead they are only allowed to use the game's preexisting scripts. (PS users also can't have any new model, texture, sound, or any other custom asset.) Meanwhile, on the PC, there's the Skyrim Script Extender that allows to not just extend the scripts like the name implies but also to directly replace the game's code, and this is used by some of the most technicallly impressive Skyrim mods out there -- stuff like Skyrim Together that adds a multiplayer functionality that the base game doesn't have, for example.