I'll take a shot at this, although it is largely a guess.

First off, Europeans (with the possible exception of a few exempted legacy things) measure all their products with the metric system. It is law there. So the road signs are all in kilometers, the liquids in liters, your height in meters, your weight in kilograms, etc.

Back when I was a kid in the late 70's and early 80's there was a similar push in the US to make the Metric system law here. I remember stories telling us that British grandmothers could adapt to distances in kilometers rather than miles, therefore we could too. And how using inches and ounces, etc. was making us uncompetitive in the world where we were the sole holdout. For whatever reason (stubbornness/sanity), that Metric system push died out, and people in the US still largely use the English system.

Now, when it comes to the gallon milk jug versus the soda bottle, I think the key is timing of their introduction. Gallon milk jugs were introduced long before the 1970's and before the metric system push. If I remember right, the 2-liter plastic bottle used by soda companies was a new product introduced right about time of that metric system push in the late 1970's. Up until then, all soda was sold either in cans or the smaller glass bottles. So the soda companies decided to introduce the 2-liter product to join the metric system push here in the US, as well as probably make manufacturing easier since then they could sell the same product worldwide.

Evan is exactly right. It's all about timing and the beauty of American stubborness regarding metric measurements.

Of course, there's probably some latent marketing value in it, too, which might explain why 2-liters are still around long after we've given up on metric. Most Americans don't know how many liters to a gallon. They just see a big jug of bubbly that looks to be about a gallon. Perceived value to the customer is greater than the real cost to the bottler.

Because it is.
Any other questions?

A gallon is about 3.8 liters... a liter and a quart are very similar in size...

When I was in 6th grade (this would have been in about 1996), we were forced to watch a series called "The Metric System." It was a sketch-comedy show for kids in which all of the skits were directly related to the metric system. In fact, the recurring villain in many of the skits was the "Inch Worm." It didn't help me any, I've lived in Canada for a year now and I'm still confused...

Canada seems to have adapted to the metric system a little more easily than the US; however, the grocery stores still advertise meat prices per pound... because that makes it sound cheaper...

and they sell milk in metric gallons, which are slightly bigger than US gallons...

Also available in Gallons at the Grocery store - water, sweet tea, juices, kool-aid, punch

What makes it even more interesting is that every other size of soft drink is in ounces. Well, except the 3 Liter (which I haven't seen lately).

Now that I think of it, I think I've seen half-liter soft drinks (16.9 ounces) and I know I've seen soft drinks in liter bottles.

The norms, however, for the smaller sizes are 12 ounce cans or bottles, 8 ounce bottles, or 20 ounce bottles.

It is beautifully stubborn, but aside from the dubious economic arguments of the 70s/80s, it did contribute towards the mars orbiter crash. (total cost > $300 million).
We haven't fully gone over to metric in the UK either, but we use it for most groceries etc. It is much easier to divide by 10 than to trying to recall whatever random number of ergs there are in a furlong, or whatever. But we still use miles for navigational distances (saves replacing all our road signs), and we still by milk and beer by the pint (liters / ml for everything else I think).
(btw, it seems more intuitive to me to buy liquids by volume than by weight, but most sodas and stuff will be the same density as water, so it doesn't make any difference).
(one more slightly rude bastion of the imperial measures: i've never heard any man quote the size of his manhood in cm, it's always inches. Interesting. centimeters would be the bigger number, although you might benefit more from a rounding error with inches.)