It's been a while since I read CoS, but the reaction Harry got when it came out that he was a parselmouth was not exactly friendly. In light of this view of parselmouths, I think that other people who have this gift would try to keep mum about it, especially a politician like Lucius. If people with it didn't tell, it could feasibly (and I'm probably really reaching here) be not so 'rare' as people think.

Animagi are supposed to be so rare, but there are only seven registered this century. We know of four unregistered in the last score of years or so, and these only those who Harry knows about.

Both abilities lose some of their tactical value if they are not kept secret, so I think the Wizarding perception their of rarity could be rather skewed.