The simple answer is that they did conquer other parts of Europe.

In Britain, the Vikings conquered Northumbria, and it's capital York, and then went on to conquer much of the island - pretty much everything from London to Chester - which they solidified as the Danelaw in a treaty with Alfred the Great. Centuries later, King Cnut actually ruled England, Norway, and Denmark from 1016 to 1035.

The vikings founded Dublin as a base for further raiding into Ireland. About a hundred years later, after being forced out of Dublin, they had a period of further settlement, founding Waterford, Cork, Wexford and Limerick.

Normandy, in France, is named after the Northmen who ruled and settled there after it was granted to Rollo after the siege of Paris in 911.

The Normans also went on to conquer England and southern Italy (though whether they could still be called Vikings at that point is a matter of some debate).

Russia (as well as Ruthenia and Belarus) gets its name from the Rus, Norsemen who settled along the Volga and other rivers of eastern Europe. The Rus, also known as the Varangians, served as mercenaries to the Byzantine Empire and some even settled in Georgia late in the Viking period.

So the Vikings definitely settled in the more fertile parts of Europe, though they usually just made up the political/military elite and so were eventually assimilated into the cultures of the places they conquered.

Iceland wasn't terribly barren at the time of its settlement in the late 9th century.

Based on pollen analysis, plant matter remains, archaeological remains, place names and written works like Landnamabok, it is estimated that approximately 25-30% of Iceland was forested at the time of settlement, a pristine area roughly the size of Belgium. An additional 30-35% were covered with wetlands, heath and alpine tundra.

Shortly after the settlement however, Icelanders adopted slash and burn techniques to create land for grazing animals and trees were chopped down to create charcoal for bog iron smelting and construction.

The most devastating impact was dealt by sheep that the settlers brought with them. The native Icelandic ecosystem lacked any large herbivores and the sudden introduction of sheep was incredibly destructive to the native forests since they would consume most birch saplings that took root and thus prevented the renewal of the forests.

By the time Landnama was written in the 12th century, the forests of Iceland are referred to in the past tense, suggesting that they were largely gone by that period. The lifespan of the Downy Birch is approximately 200 years which coincides with the near-disappearance of the Icelandic forests by the 12th century assuming little regeneration takes place.

The disappearance of the forest was by itself not a problem. Unfortunately, in the absence of any tree cover Iceland's climate and soil composition makes for a perfect storm of soil erosion (or should I perhaps say duststorm?).

Iceland's soil is classified as Andosol, volcanic soil. Volcanic soil has a lot of nice qualities, it retains water well and is rich in nutrients. It is also very loose however and will easily blow away in strong winds without plant cover, or get carried away by running water.

Iceland has plenty of running water and strong winds to support such erosion. Even worse, it has a rare type of subarctic maritime climate where the average winter is quite long, yet temperature is almost exactly at the freezing point, so freeze-thaw cycles are very common in Iceland. This results in frost erosion where saplings and large rocks get pushed out of the ground.

With regular volcanic eruptions and no birch trees that could withstand heavy ash fallout, vegetation cover was smothered and the barren areas around the volcanoes expanded. Winds blew the ash over other vegetated areas smothering them as well and expanding the desert. Colonizations by plants in the deserts were stopped by grazing sheep. A feedback loop started and tens of thousands of square kilometers of grasslands have to this day been lost to soil erosion.

Most of Iceland's lowlands once looked like this forest remnant in Northern Iceland, it is currently one of a handful of forests left in Iceland that collectively cover about 1.5% of Iceland's area.

A farm was located at this lake in the 11th century. Now the area is a desert.

I work for the Icelandic soil conservation service. Most of my sources are in Icelandic but I can recommend this English article if you are interested.

As for Greenland, keep in mind that Iceland was fully settled by the time Greenland was discovered. The prospect of more unsettled land would have been enticing in a world where there is little such land left.