Essentially, git is designed for the "bazaar" development model, while Fossil is designed for the "cathedral" one, which is what SQLite uses, being developed by just three guys working very closely together.
Thos is not the reason. The real reasons are explained quite nicely here [0] and I agree with all of them - though I still use Git because of network effects. My alternative of choice would be hg.
Essentially, git is designed for the "bazaar" development model, while Fossil is designed for the "cathedral" one, which is what SQLite uses, being developed by just three guys working very closely together.