Torrent works for some people, but many in the more than 100 countries where we have readers don't have high speed access. And even with Torrent, 4.5 GB of files is a huge amount of data to download, even with a high speed connection.
Many people, even those with high speed connections have monthly bandwidth limitations imposed by their ISPs as well.
We prefer to distribute on DVD-ROM. Almost every computer user in the world has a DVD drive, and the disk are inexpensive. $10 plus a few dollars for mailing is no impediment to anyone who wants to have the data.