The American publisher didn't except to sell more than 3,000 copies of the book in a country where nobody has seen a cathedral or studies Latin. Also, a lot of books had been written on medieval past far before him but they were not huge successes.
Because they believed, that people more liked trash novels rather than difficult readingones..
Kavya Shabu answered this
the novel the name of the rose dealt with medieval age meta physics and detective type whith very hard contexts in it people often enjoy reading simple easy to understand type novels and thats why the publisher thought it wont sell out well in america