An excellent question ! My flip answer is that Osiris was interfering. Maybe it was a risk he had to take, Sagira. But perhaps they weren’t able to engage in space? We still haven’t seen anything like Vex spaceships, unless I’m mistaken? Or - and here is where I’d bet my glimmer - the cabal had proactively engaged the vex to keep them away? They tussled extensively on Mars - I recall some grimoire from D1 (Rasputin, maybe?) where the cabal were prepared for - and anticipating - a Vex attack/materialization. Far as I know, humans don’t have that knowledge. So I wouldn’t be surprised if the Cabal were prepared.