An answer giving some good reasons for SFTP has been accepted but it seems that nobody has put forward many the advantages of using https.
First, the availabilty of tools for 'https is much wider than for SFTP. It has much wider usage, so should be a more mature technology.
It's much easier to integrate tools from different vendors without compromising on encryption strength.
Moving files around alone has little benefit. It's when the data is processed that the value is realised. It is much easier to integrate processing with 'https transfers than with SFTP.
Http (and 'https) has a vocabulary for describing how the data should be interpreted (encoding, language, mime type) and processed (get, post, put, delete) which are not available in SFTP.
Http(s) provides a mechanism for providing contextual information alongside the transfer capability - documentation, information about account usage, account management, problem logging and more which is not available is SFTP.
Often the weakest link in a secure network is the users. Users have a much better understanding of HTTPS than of SFTP. Client certificates/keys add some complexity to this but telling people to check for a green background in the URL is a lot easier than talking them through enabling strict host checking and dealing with the rollover if they ever change.
And https certificates have an expiry date, and mechanism for oob verification (HSTS pre-registration).
That you are asking the question makes me think your a security analyst / system administrator. From that viewpoint the differences might not be so clear. Go talk to your developers and users and you'll get a very different picture.