Why didn't Vikings conquered and destroyed Europe, something like Mongols did with Asia..
During the Viking age there wasn't much in the way of a unified nation concept yet, The víkingar would gather brigands, bored townsfolk or adventurous sorts and set off on their own, without the backing of any nation-state and all at the command of local warlords. There were no armies in the Viking lands, no coordination between warlords, even the Vikings adversaries didn't start to unify until around 900AD, about 120 years after the Viking Era began and the first Norsemen set out to find new lands to loot, colonize of trade with. It wasn't until 1020AD that Norway become a unified nation, transitioning from a series of petty kingdoms over the course of 150 years. Like today's Sweden, Denmark and Norway, back then populations were low in the Scandinavian countries. They didn't really start to thrive until the Black Death hit almost 370 years after the end of the Viking Age.
Too few Norsemen, no unified command, army or nation, rich lands to loot nearby and a favor for the sea rather then the continent. There were many reasons why they didn't conquer Europe. At times the Norsemen were at the command of continentals, such as Harald Hardrada who raided North Africa at the behest of the Byzantine Emperor in Palestine. To North Africans the Vikings were brutal warriors, but contrary to their reputation, to much of the rest of the Mediterranean they were merchants, bring trade not war to the distant shores. The Hagia Sophia in Istanbul, Turkey has Norse runes carved into it that predate the Varangian Guard of the Byzantine Army, whose ranks were Norsemen, showing how far and wide the Vikings would sail for trade and ties.
Instead of raiding and pillaging across Europe, the Norsemen brought trade to the Islamic world, were the first Europeans to set foot on North America, settling in Canada, and Iceland, the Faroe Islands, Greenland and Russia along the way. Norse settlements even popped up in Turkey! France's Normandy owns its name to the Vikings who settled there, Russia was helped along by the Varangians, Vikings that went Eastward, who helped bring order to the Slavic tribes of what are now Russian territories, helping to settle life down and allow a unified Russian state to form. Likewise in Georgia. Where they could the preferred exploration and trade, where they couldn't bribery or raids were the answer.
So really there's no one reason. The Vikings weren't just warriors, but renowned explorers, traders and diplomats. They didn't have one leader, but many presiding over different and sometimes warring tribes, each with their own aims or lack of them. They didn't have the population to take on Europe and wouldn't until around 1300AD when the Black Death ravage mainland Europe, but left Scandinavia's colder lands relatively unscathed. By then the Vikings had declined and a new order had begun to take from in Scandinavia.
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