The big reasons studios do remakes is precisely why they make original movies: MONEY.
In a remakes case... It's a proven FACT that the original premise/story did well enough to generate some income at the boxoffice. Hence, they assume to just duplicate the same success.
Hollywood is a RISK-AVERSE business that is all about investment and return.
And in such a fickle market place, I.E. what moviegoers actually respond to, tthey want to maximize their profits while minimizing their costs or expenses like any industry...
Hence, why the remake is heavily favored these days over original material.
There are over 20 remakes in the works right now for a major studio (I won't mention which one) out of the 37 scheduled for release (point being 17 are original scripts). How's that for statistics to back up why remakes are popular these days?
The same holds true for an adaptation for a book. If the movie bombs, the studio can just say, "Well, we don't know why it tanked. It did well AS A BOOK..."
However, what is the BIG mistake in BOTH of these presumptions?
The assumptions (pressumption) itself.
Just because an idea played well 10, 20 or even 40 years ago does not mean it will stand the test of time...
But even this doesn't stop Hollywood because now, as we all know, they don't care if they remake classics or not. They'll do remakes to B-Movie flicks as well, I.E. House on Haunted Hill; Gone in 60 Seconds, etc.
As was stated earlier, the key to a successful remake is how it is executed.
Most remakes are done for and marketed to the teen crowd, hence, why they DON'T come out that well for a number of reasons. And even the ones that are for more mature viewers, I.E. The Thomas Crown Affair, still don't live up to the originals because of the fact that (as was also stated) the stars of today are NOTHING like the stars of yesterday. They are just "actors" -- Which is DIFFERENT from being a "movie star" -- Let alone actually having any talent which is another MAJOR obstacle these days in why there is nothing good at the box office, I.E. Ashton Kutcher and others (male and female) of his ilk.