If you are asking why their wines are so popular in the US market I can offer a few reasons.

Let's separate out Germany for a moment. Their wines are popular, to a point. They are much less popular than say 4 years ago since the Euro has made them much more expensive. They retain a certain market share none the less because they really are unique. Nowhere else produces such a fine combination of crisp, minerally white with such a broad range of sweetness levels. They have that market segment pretty much to themselves, thus their ability to retain a fair share of it.

Italy and Spain on the other hand continue to be market leaders primarily because of value. Not only do they both produce a very broad array of wines, one for each palate you could say and Italy has the lead there, but they are also inexpensive, Spain's strength.

There is also a long history for Italian wines in the US which strengthens their position. And frankly both countries produce wines that are easy to drink and delicious. Italy's history however allows them to be stronger as one moves up in price. These hard times have made everyone a value shopper. Much of Spains popularity rests on the wines under $10 that they are able to produce. Italy continues to have strong sales at least up to $20 and many real values remain even up to $30.

So there you have it, the keys to their success in a nutshell.

Unique and appealing products
A history of sales in the country
Attractive pricing

Other factors certainly come into play but that is the basis for their sustained success.