Because it means having to give up control and accept that other people are going to contribute - even when they don't follow your own long-term vision.
The Elm Vision is that every library should be infinitely powerful, trivial for novices to learn, and written 100% in Elm. Any deviation from this is seen as a failure of the entire Elm project and a personal embarrassment. The problem is that these standards are way too high and essentially impossible to achieve for any nontrivial library.
The biggest risk of letting other people step is that they may succeed. They're going to write code which is incredibly useful to a lot of people, and it's going to become popular. But those developers aren't strictly following the Elm Vision. They are pragmatic, and they are willing to cut some corners in order to actually ship something. This of course leads to difficult questions like "It works great in library XYZ, why don't we just adopt that in Core Elm?". But you can't, because it breaks your Elm Vision, and you don't have infinite time and money to reinvent the wheel for dozens of core web features until it is 100% perfect according to your Vision. You're rapidly losing grip, and people start talking about a fork.
Elm is becoming a success, but your Elm Vision is rapidly degrading and at risk of failing completely. You can't let them fork. You must maintain control. Your Vision must survive, and it's the only way. You've spent so much time and effort on this. You can't let it fail. It's your life's work. You don't want all that effort to go to waste. You don't want to be a failure. We just have to make sure everyone understands the Vision, and everything will work out okay. We just have to properly educate the dissenters and all will be fine.
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I don't live in Evan's head, so I'm not going to claim this is his train of thought. But if you're looking for a reason not to let other people step in, here's one possible explanation for you.