Because most people don’t want to go to jail, pay fines, etc. Also, it tends to be good for business if you can say, convincingly, that what you’re selling is legal. Even better if you’re certified by the Better Whatever Bureau. 
This is one of those things that will not simply disappear in a puff of smoke, but as more and more jurisdictions ban it, and more horror stories are publicized, it will become less popular and harder to find.
For example, these days in western countries, if you want to have your daughter infibulated (you probably know what that means, but 
for those who don’t), you may be able to find someone within the country who will do it, but the illegality of it will scare away X% of customers. You may be able to go overseas for it, but international travel is 麻煩 even in good times, it costs money, and even if you’re backward enough to want it in the first place, you might still be turned off by the backward state of medicine in whichever country, plus it may be illegal to take a child out the country you’re already in for the purpose of having her infibulated (as for example with Canada’s proposed CT ban)… and so on.
If you show me statistics indicating that infibulation is on the rise worldwide, I will reconsider what I just wrote. It probably has risen over the last X decades in western countries, due to immigration, but then among those immigrants it will of course go down the longer they stay. If it were legal, that decline would most likely be slower.
Then what are we arguing about?
Strictly speaking, the law – the Criminal Code – already exists, and C-8 proposes an amendment to address a shortcoming in this existing law.
Maybe you wouldn’t care. I would. That girl in Taipei who got decapitated by a maniac on the street, we can say there was a reason for it (the guy was insane), but we can also say it was a random event, and while it does remind us all to be vigilant in general and perhaps inspires people to change the public policy on mental health, it’s not indicative of a movement of maniacs setting out to decapitate as many children as they possibly can.
In contrast, there are known to be organized groups that want to assault and/or murder as many X, Y or Z as they possibly can, and once you notice a pattern like that, it may be prudent to try something other than business as usual.
I think it’s here and there. Without an explicit ban, people will use concepts like freedom of religion and the sanctity of the family in their defense, and while the Better Whatever Bureau may refuse to certify anyone who does it, the Better Whichever Bureau can fill the gap.
Among those who believe in CT, most will not say please take my money and do obviously illegal things to my child. Instead they’ll say things like it’s not obviously illegal – in fact maybe it’s not illegal at all – I mean how can it be kidnapping when there’s parental consent? and so on. If you close the curtains but leave the window open, you still have an open window, and every now and then a storm will remind you of that fact.
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What’s next? Gee, idunno. Ban crack and fentanyl? Taxes and warning labels on cigarettes and alcohol?
Oops, already happened, and the world didn’t end. The state (in the broad sense) has always banned this thing and made that thing difficult. If you want absolute freedom, you’ll never find it, because you need anarchy for that, and true anarchy doesn’t exist. -
Under the proposed ban, you can still perform CT on a consenting adult if you only advertise by word of mouth and do it for free. Yes, that will have a chilling effect. Yes, it’s intentional (like taxes on cigarettes and alcohol).
It’s still CT whether the thing to be converted is sexual orientation or gender identity.
You know those psychic hotline ads that come with the for entertainment purposes only disclaimer? 
