It's the recursive nature of it.

Almost any program can be written in a DSL that solves the class of problem that the program solves. So if you consider the user stories of your project as that class of problem, you can come up with with a DSL (mostly in form of pseudocode) that can describe the solution. But first you need to refine the terms down to some primitives (context free grammar). You then need to think about the implementation of the execution machine. which will be a graph of states (automata). The transition between the states will be driven by an execution machine. The latter needs not be as basic as the Turing machine.

But often, you do not need to do all these stuff. You can just use common abstractions like design patterns, data structures, and basic algorithms to have a ready made solution. But you still have to compose them and if you understand how everything works, it's easier to do so.