Because suddenly the oil isn't Russian anymore but Indian or something like that. Here's[1] an article going into how after the 2014 sanctions suddenly a lot of tropical fruits started growing in Belarussia, or how the landlocked country suddenly started exporting tons of Belarusian shrimp.

[1]: https://east-center.org/wp-content/uploads/2018/01/Belarus-E...

Europe kept buying Russian oil and gas, because other sources could not come online quickly enough. Tariffs would have only made energy even more expensive than it already was. Now it's mostly Hungary and Slovakia buying Russian energy, as well as some LNG imports. Those two countries are not too keen on sanctioning Russia, especially in ways that would hurt their economy. Any attempts to impose tariffs on energy imports from Russia would have led to a major internal crisis in the EU.

> Why not tariffs? Basically the continuous version of discrete sanctions

There are still some qualitative differences: With sanctions, you'll know something dodgy has occurred when you find a pallet of My Little Putin dolls traveling through the port, you don't need to call up a bunch of lawyers and accountants.

That said, I readily admit that oil is a lot more disguise-able and fungible.