If the game were open-source, that means that forks projects would occur, some prioritizing immediate gains in appeal/playability by sacrificing the long term (virtually the opposite of Toady's approach of extremely-long-term planning), driving attention away from the main project, from Bay12Games, and quite likely resulting in less donation money coming in.
This is in addition to what MrWiggles said, which is basically true. While I personally think that Toady having a couple "team members" would be a good idea, in terms of either grunt/busywork or project management or high-level complicated programming and junk, because no one person is great at everything, so when you have a one-person team, that one person's weaknesses tend to cause more problems than they should, especially with such an ambitious project. However, Toady emphatically doesn't enjoy working well with others, and his long-term comfort level with the project is of the utmost importance considering how long-term the project is; he
needs to maintain his own interest above practically any other concern. Ideally, he wouldn't mind working with others as much, but no person or situation is ideal. That being said, I don't think being open-source would ever be a good idea for the project unless Toady is very, very much done with it for good.
If the donations stopped coming in as generous as they are today, then he would go get another job, possibly full time, and work on DF and other Bay12 projects in his spare times, as he and his brother have done their entire life.
Except he obviously doesn't want to do that, or else he
would be doing it. There's a reason he
used to do it that way. He prefers being able to make a living off the game because it affords him the opportunity to do what he enjoys doing full-time. Securing his own livelihood is a definite factor and has been in the past.