We developed an SFTP server instead of FTP server because we believe SFTP is a better and more powerful protocol that is going to stay in use for decades. We are not so sure about FTP, which is a legacy protocol with a design that limits its usefulness and makes it notoriously firewall-unfriendly.
Also, having been in the .NET component industry since 2003, we have been observing slowly-but-steadily diminishing interest in FTP compared to SFTP. Google Trends confirm this observation: FTP vs SFTP
This said, we have been considering adding FTP server support to our component portfolio, because FTP is still useful in many scenarios, particularly those involving legacy systems. It's just not very high on our list of priorities at the moment.