They should focus on one book .

Agreed, also that's the best goosebumps book by far

Not sure if they would have enough material to make a feature length film out of one chapter book.

They originally did think of that, but problems arose. Which book should they choose? Can one book make a movie? Should it be an anthology? All those questions came to mind until they decided to come up with a new story so they could use as many monsters as they wanted. This was by far the better route, especially since many different things could happen for the sequels.