Moshe's command was to spare the maidens. (Bamidbar 31:18)

The Gemara also brings up the idea that a female convert 3 years and younger can marry a Kohen. Perhaps that has something to do with it?

And our King David left no man, woman, or child alive on various occasions. This may have been in a similar vein to the precedent set by Moshe, (e.g. everyone dies except the maidens/yafos toar etc.)

Moshe's command was to spare the maidens, (Bamidbar 31:18) but not the males. Since Moshe was an accepted prophet, there is a positive mitzvah to heed his words.

The Gemara also brings up the idea (although it is not accepted) that a female convert 3 years and younger can marry a Kohen. Perhaps that has something to do with it?

And our King David left no man, woman, or child alive on various occasions. This may have been in a similar vein to the precedent set by Moshe, (e.g. everyone dies except the maidens/yafos toar etc.)

Moshe's command was to spare the maidens. (Bamidbar 31:18)

The gemara also brings up the idea that a female convert 3 years and younger can marry a cohen. Perhaps that has something to do with it?

And our King David left no man, woman, or child alive on various occasions. This may have been in a similar vein to the precedent set by Moshe, (i.e. everyone dies except the maidens/yafos toar etc. etc.)

Moshe's command was to spare the maidens. (Bamidbar 31:18)

The Gemara also brings up the idea that a female convert 3 years and younger can marry a Kohen. Perhaps that has something to do with it?

And our King David left no man, woman, or child alive on various occasions. This may have been in a similar vein to the precedent set by Moshe, (e.g. everyone dies except the maidens/yafos toar etc.)