The prevalence of `var` statements in the TypeScript codebase is attributed to performance considerations related to the Temporal Dead Zone (TDZ) associated with `let` and `const` declarations. While TDZ helps prevent accessing uninitialized variables and improves code safety, it introduces overhead that impacts performance, leading TypeScript developers to favor `var` for better efficiency. Ultimately, this choice reflects the balance between code safety and execution speed in the TypeScript environment.