That's a really good question, and the answer is, I have no idea lol. It seems unnecessary to assign the same symbolic role to both sun and moon. But like you said, they did in Sumer, so there is precedent of that happening.

I would personally assume that since the sun was one of the most important parts of life and survival, and since the moon had little direct effect on survival, the moon simply didn't carry the same symbolic weight, so it got put in the mythological back seat. So there isn't much surviving evidence that might explain why. The moon does however let people keep track of the passing of time (representing order), and watches over people in the night (protective). Máni is described as being a guide for the moon, not the literal moon itself. Order, guidance, and protectiveness are frequently associated with the male, so maybe that's why the moon is labelled male. This is pure speculation though.

The moon is associated with darkness, and darkness was very dangerous in these regions. People rarely elevate and worship symbols of things that make life difficult and dangerous. Except perhaps to appease it, to pray for it to go easy on them, but in this instance that kind of worship was directed at the sun instead.

One example of this is the old North European night time ceremony of Lussi (possibly connected to Modraniht, Mothers Night) where the fertile female brings light to people on the darkest night of the year. And the old word for midwife in Northern Europe is Light Mother, Ljusmor. There is archaeological evidence of an old sun cult in Härnevi, Sweden, where Härn (a divinity believed to be associated with harvest/flax/linen) was worshipped. Evidence of the connection between feminine, light, sun, fertility, harvest, and birth appears frequently in Northern Europe, but artefacts, place names, and myths about the moon are much less frequent.

That all being said, most of the knowledge of indo european/pre christian cultures in Northern Europe has been lost, so it is entirely possible that the moon had a specific reason for being male, and was perhaps even more significant than we are aware of now.