Because it moves the onus of dealing with the fraud on the bank, not the seller.
Let's say you sell a premium account on SuperSite, giving for 10€ one month of free movie streaming. Now you get a good deal with Publisher, which gives you access to their movies catalog.
Suddenly you get users registering, subscribing, watching the movie they want, then cancelling through their bank, by doing a chargeback, claiming fraud or that you didn't deliver the product. Sometimes, it was even a stolen card, and the user doing the chargeback is honest.
Without 3D Secure, you're out of your money until the issue is resolved, which can take up to 60 days. With 3D Secure, you keep the money, only after 60 days, if you have been found to be the one in the wrong, do you give back the money. And if it's a stolen card, since it passed 3D Secure it still isn't your problem in a lot of case.
That is the incentive they used to get merchants to deploy it quickly and in just a few years it moved from nowhere to everywhere in France and throughout the EU.
We've seen conversion rates with 3D secure for certain products drop up to 40%.
You need to tradeoff fraud/dispute stuff vs usability/conversions.