why can't I download Battlefield 1942

Unfortunately, BF 1942 servers have been shut off, and the game itself, taken out of Origin. 

You can read more about it, here:http://answers.ea.com/t5/Battlefield-1942/June-30th-2014-Online-Shutdown-for-Battlefield-1942/m-p/2971266#U2971266

Also, if you ever need help regarding Battlefield 1942, please kindly post on this section of the forums:http://answers.ea.com/t5/Battlefield-1942/bd-p/battlefield-1942