The Romans’ only interest was in land troops, whose spirit was always to hold firm, to fight in the same place and to die there. They could have no esteem for the practice of seamen who show up for a fight, flee, return, constantly elude danger, make use of ruse and rarely force. None of that was in the genius of the Greeks  and even less that of the Romans.
Therefore they destined for the marine only men who were not citizens of sufficient stature to occupy a position in the legions ; seamen were usually freed slaves. 
We have today neither the same esteem for land troops nor the same disdain for those at sea. The art of the former is diminished ; that of the latter is increased  ; but we esteem things in proportion to the degree of ability required to do them well.