While it could bring enjoyment to some, allowing for file changes could have many consequences that could harm players' enjoyment of the game and the fairness of the matchmaking and leaderboards.
For example with a file change a person could turn a wall invisible to see another player through it, or create a sound that tells them about the item respawn time or that tells them the position of the other players more easily.
It could also result in the game looking/sounding very different from one video/stream to another as certain changes could make the game unrecognizable and make it confusing for viewers, potentially making the game look/sound less appealing for possible new players that might have tried the game after watching it.
If someone were to add a copyrighted sound in Diabotical it increases the potential of some trouble if the copyright owner thought that this was something available in the game by default.
While some restrictions could be employed while still allowing for custom assets to be used, Diabotical is a free game with a cosmetic in-game sales model so it is important to have some control over the customization options, so that the development team can receive financial support from the players and continue developing/improving the game for years to come.
It is not impossible that some kind of testing environment could be created that would allow for changes like different sounds, if someone wanted to create an announcer pack, or create a new flower pot model to be available in the map editor, or maybe even test changes to a mode, but this would have to be a closed environment in some ways that wouldn't interfere with other players.