"Why" would be that somebody created one without noticing the other existed. That's unfortunate. We should try to figure out how to detect and prevent those. But right now this can happen and we rely on people noticing.
Moderators can merge tags, so when you see cases like this, please flag a post containing one of them (since you can't flag a tag directly) and someone will take a look. I suggest that for counting nouns we normalize on the plurals; that feels more natural to me, and you're probably cooking more than one clam anyway, right? (This suggestion does not apply to mass nouns, like "seafood" or "curry".)
I have merged [clam] into [clams].