The first bank to only authorize purchases with 3DSecure lowers the fraud rate a lot, but people with multiple cards will mostly choose to use other cards, because it's less friction.
Anyway, the merchant eats fraud for card not present transactions. So why would the bank choose to reduce its payment volume in order to reduce fraud it doesn't even have to pay for?
If the merchant says 3D Secure only, it reduces fraud, but also reduces payment volume, because most customers will choose to use a merchant with less friction, especially if their issuing bank doesn't do 3D Secure, or it's broken when they go to purchase.
Reducing fraud is good for merchants, but it the drop in sales may not be worth it. There's a lot of other things merchants can do to reduce fraud that aren't likely to cut into sales as much.