Main reason: Level of detailing in his books is too good... Each book delves into the subject chosen in a nice and deep manner.. esp. hotel, overload, money changers ,etc...
btw, did you read Flight into danger ? Hailey wrote it jointly with another guy... This also is one short and sweet , quick paced book.
oh yeah.. Hardy boys! brings back memories.. those days no online books /etc... buying books also was out of question, so have to depend on few libraries in our city... Incidentally, the initial few hardy boys were way better ..ie, first 70 or so... the latter ones were too boring...
Nancy drew is also similar, but didn't become so popular
Now I have a whole collection of hardy boy's in kindle format, which I am giving to my kid to read....