Primarily because, until you mentioned it now, I wasn't even aware it was an option!
That said, I generally shy away from shallow clones and probably wouldn't use it here:
- it's a trap for people who ever want to work in that repo normally (we use the trick for more than just LLVM)
- I believe shallow clones, over time (e.g. for contributors), are less nefficient than deep clones - I would expect shallow cloning to reuse fewer objects and benefit less from git's design. [0] describes a historic issue on this topic
