Because in this world, it was an honour to fight for your people, and it was supposed to be seen as an even greater honour to die for them. It feels sort of like a comparison to the Vietnam war – at that point, if you were a draft-dodger you were a coward. At least outside of a group of people who were also supportive of draft-dodging. Nowadays we have more choice, we have an opportunity to decide if fighting for our country is something worth it’s honour to us. For Rumpel? He was probably the only man in his village who refused to fight, so while all these other women were able to remember their husbands as brave soldiers who fought to protect them and their children, Milha is the only one who has a husband who did not give that sacrifice.
So Rumpel, for that reason, isn’t a coward by OUR standards, but by the standards of HIS world and time he most certainly was a coward. And deserting Bae made him a coward. AND telling Baelfire that Milha had died. Remember when he leaves the ship he asks “What do I tell my boy?” And Hook says something along the lines of “What any honourable man would, the truth.” But Rumpel doesn’t do that, because he’s afraid of what the truth means.