On our servers, that's over 100gb times the amount of users that download it, in the order of hundreds of terabytes. We're a small company, and bandwidth costs us.
Not to mention that RealTracks for Windows are distributed using EXE files, which have a hard 2gb limit. 100gb split into 2gb files would be a pain at 50+ files to install. 1.5tb split into 2gb files would be a nightmare at 750+ files. The Mac DMG files don't have such a limit, so they're easier to deal with, but bandwidth costs and supporting Windows users with 750+ files are still some of the factors we have to consider. There are additional factors past these two as well, but packing and distributing are two of the larger ones.