Because you only intend to run it once, on one input, and ad-hoc interpreters are usually simpler to write than compilers.

Such a one-off Haskell interpreter might even get away without or with less rigorous type checking.

An interpreter omits the target architecture/format question and thus is more portable than a compiler.

Of course, you could compile to a higher-level language, for instance directly to C. But that's usually still a complication that only gives you performance. And performance is something you probably don't need during bootstrapping.