Most likely, Sony studios wanted to capitalize on a character who was part of a movie that was successful and made a lot of money in the movie theaters. That movie was Spiderman. This movie was so successful that it became a franchise with a series of movies. Now, Sony wanted to add to this series by creating a movie about the antagonist which is Venom.
Any movie that is made about a Marvel comics book character will be successful because the movies based on comic books are successful. People relate and know about this character before they even see the movie. However, Sony will have to wait until the movie comes into the theaters to determine if the movie will be successful.