because the linux build is the whole game, whereas the others are just the launchers which download and install the game.
Fortunately, there are community-made launchers. Well, one of them is actually from an Uber Dev. They are: UberWillam's patcher based on Go; Pyrus' patcher based on Python; DeathByDenim's patcher based on Qt5. The first two are command-line based, the third is graphical (and made by me!).
Linux build also include big part of Steam Runtime as it's needed to make game work on distributions other that Ubuntu 12.04.