Hello,
Nikola has given most of the whole picture. Without speaking about new cards, challenges or game mechanics, scripting or anything else in technical level, allowing custom contents must be tightly controlled.
If everybody were honest and if custom contents would stay in closed friend circles, it would effectively be quite fun to play with custom boards.
However, the problem is that, sadly, everybody isn't honest, and even with the best honesty of the world, by the play of sharing from friend to friend of friend and so on, the so hilarious but politically incorrect content of your group of friends will eventually hurt someone and/or fall into the hands of a troublemaker one day. The question isn't whether or not, but when it's going to happen.
At the end the complain come to us and it's quite a difficult position to just say well, it's user created content, this isn't our fault, we didn't created this content and don't support it, we can't do anything against it.
Even if we find a responsible to ban or punish somehow, it's too late: the content will keep spreading out.
In our era of politically correct world, so sad it could be, we can't take any risk
You can submit your creations to us, but a review is an absolute must before allowing it to be used by everybody.
That's why we have a team dedicated for the quiz, for example, and if we want to allow submissions from players in other games, we need to mount similar teams.
I hope it's clearer now.
Note, you can apply the same reasoning to custom cards against humanity packs.