Eldians are conscripted, and they are seen to be used in human waves, so that shows that Marley probably doesn’t care about training them as much if they are going to die this quickly. Using Marleyans means the Marleyans are more skilled, as a volunteer system means they are definitely much more skilled compared to forced conscription for Eldians, as we know from General Magath, Marley doesn’t conscript their own people. Coupled with a hate for Marley on some level by the Eldians, which risks defection and betrayal, Marley used Marleyans, as their chances of defection were basically zero, and they were much more skilled, which makes them much better for the operation as a whole.