That's because, in the United States, 3G networks really suck. In Europe, some telecom providers offer up to 28.8 Mbps over 3G networks (which results in something like 20 Mbps of real world speeds) which is really fast enough. In the US, coverage isn't always great: I have seen videos where 3G speeds in the United States are slower than EDGE speeds are here in Europe.
LTE will be great to use in the future but right now it is far from a necessity.
This is because European countries are tiny compared to the whole of the US. Creating a 3g net over Portugal is a fraction of the cost of what it takes in the US, as Towers can be closer together as the populous is bunched in higher ratios per square kilometer.
By the way, not all Euro countries are the same. 3G speeds on the iPhone all over England sucked.