Why is my vegetable stock bitter, but the chicken stock not?
I notice that your recipe doesn't include any salt. That's important, because salt decreases the sensation of bitterness. Chicken contains a certain amount of salt, and I suspect that's making the difference. (The "umami" -- brothy -- taste of chicken may also decrease the sensation of bitterness, though as I understand it there's still some disagreement about that.)

Try mixing 1/8 tsp of salt into one cup of your vegetable stock as a test. I suspect that'll decrease the bitterness to a comparable level.

Oh, and if you want to make your stock less bitter without making it more salty, use parsley stems and leaves instead of parsley root, and celery (including leaves) instead of celeriac. Those two roots will be the primary sources of bitterness.

Incidentally, I very much approve of you not salting your original stock, and instead salting whatever you use it in. Unsalted stock is more flexible, and is more forgiving if you decide you need to concentrate it.