Because you would need to bootstrap it every time you want to run it, since Javascript is interpreted. This means you'd need a 3rd party Javascript interpreter to run your Javascript interpreter.
They would have had to make their compiler provide a batch compilation mode in addition to the incremental compilation mode, but that should not be a pain point. After all, batch compiling is easier to do than incremental compiling.
That's a whole lot of unnecessary -- and nontrivial -- work, when all you want to do is interpret Javascript.
When you build V8 from source, you can have it generate a snapshot of the memory state once the libraries are loaded and this gets packaged in the executable. It makes it slightly bigger but the load time is blazingly fast.
There's certainly extra work in this approach, but maybe it would be worth it if it allows the compiler to be written in a more productive language.