The short answer is of course that the vikings had a strong naval tradition. The sea and sailing was very important to the Vikings for several reasons. Fishing was an important source of food but sailing was an absolute nessecity for trade or travel. Scandinavia was vast, largely consisting of rough terrain and relatively sparesly populated compared to the rest of Europe at the time. If you wanted to get somewhere you got there by boat. This of course fostered a strong naval tradition by nessectiy. Added to this, almost every major settlement in Scandinavia was by the sea or a major waterway. People lived very close to the sea.

This led to relatively rapid naval architectural and navigational inventions. A few of the most famous ones include the longship, the half wheel and the sunstone which, along with a lifetime of developing a sense of currents, speeds and distances traveled, allowed the Vikings to navigate very accurately for the time. (to those unfamiliar with sunstones and half wheels, they are navigational aids. A sunstone is a type of quartz that polarizes the light. If held up against the sky. or over a flat surface, it will refract the light differently depending on how high the sun is in the sky causing it to take on different colours. This allows you to roughly gauge how high the sun is in the sky even if it's cloudy. A half wheel is a kind of sun-diameter which allows you to determine latitude and compass heading to within about 5 degrees).

A longship is a fast and versatile ship, relatively sea worthy for the time (especially the somewhat heavier norwegian models) and with a very shallow draft which allowed navigation of rivers inaccessible to heavier types of ships. A longship can reach speeds up to 15 knots which was unmatched at the time (though a more common speed was probably 5-10 knots). The versatility of the longship combined with the relatively advanced navigation is what enabled the vikings to raid, trade and settle such vast areas.

If you want more specific information you'll have to ask a more specific question. "good sailors" is after all a relative term. They vikings did not for example do battle at sea to any great degree and when they did, longships were usually used more or less as platforms for infantry combat.

While their skill as sailors is without a doubt what I think is most important is the difference in technology. Throughout Europe during that time most of European naval technology was effectively derived from Roman galleys and other Mediterranean craft. These medieval craft were large, cumbersome, and generally poorly suited for atlantic travel, but excelled in the protected waters of the Mediterranean. The Viking longboat on the other hand was well suited for atlantic travel. They featured a deep keelboard to provide stability, a relatively narrow silhouette which provided a much faster trip, and most importantly they were designed to be able to take waves over the deck and drain the water effectively. Additionally the boat was so narrow it could be taken down rivers and light enough to be beached without damaging the hull. There are even stories with longboats being portaged by their crew.

Simply put the medieval world was not technologically equipped to travel on the open atlantic, but rather relied upon ancient galley designs that were meant to ply the calm waters of the Mediterranean.

So yes the Vikings were impressive sailors, but they became legendary because they had such vastly superior naval technology compared to their peers.