I don't have any inside information about the game, but here are some general reasons why not to enable modding unless it's economically feasible to support it:
Modding support without full source code release generally requires you to make reasonable abstractions while you are writing the game whereby there will be a large and meaningful portion of content available in an intelligible fashion for extension, addition, modification, and so forth. If you don't do that because it's easier to create content that's entangled with your source code, then either you have to spend a lot of time refactoring your work to make it more transparent, or you have to open your source code for modders.
Modding support with full source code risks the possibility of some jerk posting a copy of your game on his own site. Or (more likely) some modder spends some number of hundreds of hours on his mod (which is a full version of the game) and then makes it available to owners of the game, except due to hiccups with Steam Workshop, it's not available. So then copies start circulating on the broader Internet, available not just to owners but everyone. You can get an injunction and pay a law firm some amount of money and get it taken down, and you can generate a lot of ill-will because you're fighting against some dev who spent a lot of time making your game more awesome. Or you can just choose not to have this problem. "Illegal copies of an electronic game? My word!," you may say, dropping your monocle. "What is this world coming to? The next thing you know, they'll invent a way to illegally download CD music!" And, sure, piracy is always an option for customers, but as a developer you don't want to make decisions that could cause illegal copies to be the only option for reaching desirable content.
Modding also increases the number of bug reports that come in, because users can't differentiate between mod bugs and base game bugs. Or (alternatively) mods expose bugs that really were always there in the base game, but were never expressed in the content you had. The average user isn't going to mention that he's using the Romanceable Brothers Mod when he's complaining about why your programmers (he thinks you have more than one) couldn't eliminate crashes in five years of development time. He's just going to leave a negative review on Steam about the noob fucks (again, he thinks you have more than one noob fuck) at Overhype.
Also, if your programmer, in a fit of ill-advised altruism, should be so bold and so foolish as to make himself accessible to the modders, his inbox is going to be stuffed full of abstract questions about code he wrote 1-5 years ago. At the very least he needs to document instructions about how to build the thing, which might be as easy as "clone repository, press button" and might be a 39-step process starting with "So first, you're going to have to recompile these utilities. Also, heads up, you're going to need a remote laptop on your network named b1487." One-man dev teams, especially when the one man is a virtuoso programmer, tend to do really fucking weird things without even noticing them, which will likely astound everyone else in the world. This applies not just to your programmer, but the sometimes wonderful and sometimes insane programmers who are fans of your game and want to mod it. Does your game compile on Red Hat distributions from 2003? You may not know the answer to this question, but some helpful fan of yours in Central Asia may decide to write to tell you the answer. (Spoilers: it doesn't.)
You get grumbly modders (who had previously been excited product advocates) if he ignores them, and you get a huge hit to that programmer's productivity if he does. "Oh, but you can just disclaim that you won't provide support for modders!", you think. That'll probably work really well, because if there's one thing that diehard fans of PC games are known for, it's how reasonable they are.
When all's said and done, Overhype will have grossed $2-3m from this game that took them five years to make, which (after paying Steam, artists, composers, office space if they decided to have an office, any promotional activities like showing up at conventions, dues to terrible quasi-racketeering business entities prevalent in Germany that are prereqs for participating in the above) works out to some mid-high five-figure amount per Overhype bro per year. This would make it what we call a labor of love, as differentiated from the type of labor where you get paid a market rate for the work you do. In their shoes, I would also want to move on.