Because that's just how God wanted things to be :)
Lonely Planet was created in 1972.
On This Planet was created in 1997.
favor has a long a and planet has a short a sound.
Because space is a Vacuum, there are not enough gas molecules around to transfer the sound waves.
Planet Fabulous was created in 1994.
Cannabis Planet was created in 2009.
Planet Love was created in 2000.
The Loneliest Planet was created in 2011.
Planet News was created in 1971.
Planet of the Wolves was created in 1996.
Raging Planet was created in 1998.