Because God said otherwise.
Genesis 17:10-14
11 And ye shall be circumcised in the flesh of your foreskin; and it shall be a token of a covenant betwixt Me and you. 12 And he that is eight days old shall be circumcised among you, every male throughout your generations, he that is born in the house, or bought with money of any foreigner, that is not of thy seed.
(my emphasis)
Leviticus 12:3 And in the eighth day the flesh of his foreskin shall be circumcised.
There is a lot of room and debate for a lot of the Laws in the bible - this one is pretty darn specific and clear.
Slavery (as per your example) was neither condemned nor actually said was the preferred economy God desired. God did appear to give a lot of laws and rules on how ones slaves are to be gotten, who could and could not be sold into slavery and even the rights and responsibilities of master to slave/slave to master.
No where does God say 'Have slaves or else you are cut off'.....
I note that that weblink you gave specified 'ritual circumcision' That appears to be directed at Jewish people and Jewish people only. If Germany were to poise such a law we wouldn't hear the end of it until every Nazi comment ever invented had been rehashed.
While it happens in Islam, few people know that and only think of 'The Jews' when its brought up.
In today's political climate I have to wonder at the sanity of the Netherlands with making such an open attack on two of the three primary religions that strikes the root of their religion.
While people may not like the idea of little boys having their foreskin snipped off, I fear its been happening for thousands of years and we already have one religious group pretty ticked off at western governments to the point where individuals fly planes into buildings.
I would hate to tick off the Jews - last I heard they had the Atomic Bomb.