FTP has its origins at M.I.T. around 1971, and while it has evolved somewhat over the years, it's one of the oldest protocols that is still in use and dates back to the days of Unix mainframe computers and dumb terminals. For example, a student at a university would log on to their shell on a campus server with their terminal in the computer lab and invoke an FTP client program to connect to another computer and download course curriculum or research documents, to their home directory. If you're not familiar with FTP, chances are you've used it many times without even giving it a second thought. Quite simply, it is still widely used because it is the most stable and reliable protocol for transferring files over the Internet. This means your download has a greater chance of completing without transfer errors, and intact without corruption.
Standards are what make platform independent technologies like the Internet possible.
This rationale ensures reliable communications. FTP also uses less overhead than other file transfer mechanisms, with fewer packets being sent back and forth to perform the file transfer. The main reason for this is that FTP is able to download files in binary form. Basically, that means your file is encoded as text in the data stream, and converted back to binary on your end. The encoding adds considerable overhead to the transmission.