With eval language designers might have a delayed execution that could make compile-time optimizations less straightforward. Part of the toolset could have to be present during deployment time to be able to handle eval which happens at runtime. And yes, security concerns could also play a hand.
In Dart the code declaration is taken very seriously in many ways, and avoiding eval buys them more compile-time, loading-time and perhaps runtime optimization opportunities. I find that more than the language designers themselves, it's the end-users or developers that ask for more "bondage and discipline" from their tools. People who come from Java and C++ backgrounds, for example. They just can't have enough compile-time stuff aided by an IDE.
Not including eval seems to be a "discipline and bondage" fetish of language designers.
Yes, you can write pretty awful code with eval. But it allows metaprogramming aswell. I'd like to judge if something is awful or awesome.