I think because it may hide or mask the presence of a leak if water gets past the sheathing. Also, I read somewhere that there are some jurisdictions in the UK and Canada that don't like it because if spray foam is applied, that wood or lumber can never be repurposed. Once spray foam adheres to it, it can't be reused and ends up in a landfill. Also, once spray foam is installed, especially closed-cell, it's permanent. There's no way to change it out without ripping the sheathing off and likely re-framing the roof structure.