You can change project settings later on, but this is for an initial value for the project. Because tempo is a global variable and affects all channels, it wouldn't make sense to put it under instrument or channel settings. I could add something that will let you change the tempo somewhere within the song but I hadn't initially planned it.
Besides, you can still adjust notes to the tempo, and I myself can't see many cases where I might want to change tempo at a specific point in a song... If it would be an important feature it won't be difficult to implement though :D