Adding mods into the core game comes with way more than just an IP issue.
First up many mods might "work" but won't necessarily work for all users, so there might be adjustments before a mod can go for the general public release.
Then there's the issue that a mod might rely on code from more than one person. This might be a clear cut situation or it might be a tricky mess where bits of code are owned by different people, some of which might not be involved with that mod specifically (its generic code made for a function that the modder has used).
It might even be that he mod is actually based on a different mod by a different modder and adjusted to suit its new purpose.
So the ownership of code and the legalities of using it can become a minefield for a developer.
Then there's the issue of the floodgate aspect. Letting one modder's work in can result in loads more screaming at favouritism if their mod isn't added.
There can also be issues with contracts and pay within the company. There might be restrictions on the firm regarding the use of 3rd party code within their software. It might also be that if the modder's code is used in a commercial game then the modder is due pay by the company. Suddenly it gets really messy and complicated!
In general developers are often cautious of incorporating modder content into their games; unless its a feature designed from the outset of the project. Or one where all major investors/stake holders within the firm are quickly on board with the idea and want to push forward even with the risks/additional legwork etc...
In other news - Gladius - where are my four legged necrons - where are my three legged necrons?!