The specific reason would depend on the ones given by the organization or entity that banned it. In general, conservative Christian groups would want it banned because it makes light of God and the Creation story.
The "Hitchhiker's Guide to the Galaxy" has not been banned in general, but it has faced occasional challenges in certain schools or libraries primarily due to its irreverent and satirical content, as well as some instances of language and topics that some may find inappropriate for younger readers.
The question is impossible to answer without knowing who or what specific organization banned the book. Thousands of books get "banned" by various groups every year, and rarely do such acts have any significance to the general public. You can still get any book you like from a bookstore and/or from 99% of the public libraries out there, regardless of what silly powerless group chose to enact a "ban."
Space
The Hitch-Hiker's Guide to the Galaxy is 100% fact in the 42nd dimension.
Shynola
Approximately 15% of schools list The Hitchhiker's Guide to the Galaxy in their curriculum.
hitchhikers guide to the galaxy... it's a hilarious movie
Because it says so in the hitchhikers guide to the galaxy.
2
The brilliant book does not tell you, that's the point of the book!
for leaving just before the world exploded in the hitchhikers guide to the galaxy
The "trilogy" of five books were published between 1979 and 1992.
Hit 4-2 in that order. It's from The Hitchhikers Guide to The Galaxy.
The Earth is destroyed to make way for a new hyperspace bypass.