The detailed answer to this is the subject of a lifetime of studying history. The TL;DR-version would be "Superior means of production and superior numbers". In the long run, a war between two (roughly) technologically equal parties comes down to a pissing contest of national economy. The allies had the bigger bladder.

The bulk of the economic cost of the Axis war machine was borne by Germany and paid for using every dirty trick in every book on economics ever written. From 1933 an onwards Germany was constantly balancing on the edge of economic breakdown. Had anyone outside of Germany had the insight we have today into German Economics they would have known that a war of expansion was inevitable, it was quite literally that or complete economic collapse.

Had Germany not enjoyed the early success it did (Czechoslovakia, Austria, Sudetenland and subsequently Poland and France) the German economy would probably have collapsed on its own accord. This was well known by 1939 or so. However, most of the Allies were still gearing up for a war of attrition at this point, knowing that Germany would be unable to sustain it for very long they were rightfully optimistic until France fell.

By 1938 or so the only option for Germany was an aggressive war of expansion, nothing else could have justified the measures that needed to be taken to continue the rearmament and sustain the armed forces. If you want to read more on this topic I highly recommend Adam Tooze's Wages of Destruction: The Making and Breaking of the Nazi Economy.

The primary reason the Allies won was because of industry. By the end of World War 2 America was 50% of the World's GDP. Germany and Japan may have had technology but the allies had shear numbers and resources that they could throw at the Axis. This coupled with the fact that Germany was running on synthetic oil and had failed to capture any substantial oilfields which would allow it continue the fight and the fact that the allies could actually afford to produce more the three Sherman tanks needed to take on one Panther in the field. In addition to this there was the absolute terrible bureaucracy in the German government at the time. Hitler himself was reported to get up at around 11am and he designed that it was a good idea manage the war effort himself and didn't follow the advice of his military advisors. Had the German military not attacked the Soviet Union and put more funds into the Me262 program and the other jet powered aircraft programs they could have done much better but still they wouldn't have won.