There are several crucial factors which led to his rise to power.
Several historians have argued that the Great Depression led to his eventual rise and the defrachising of the German populous towards the Weimar Government. Hence it can be argued that National Socialist (the Nazis) came to power more because of their enemies’ weakness and failures than because of their own strengths.
Other historians has said of Hitler and Nazi Party's ideology and promises made to the Germans that made them so popular in the elections. In the period 1930-1935, Nazi party saw a constant and steep rise in votes for them, demonstrating their popularity of their Nationalist movement.
Another major factor would be Hitler's personal charisma and political wit. Afterall, Hindenburg (who was the President in 1933) could have vetoed Nazi parties movement for power, and the communist party saw votes increasing as well between 1929-1933. What led to the exponential increase of support of Hitler can thus be said to be arising from his personality. Hindenburg and the German political elite needed someone charismatic like Hitler to unite the masses, while at the same time, Hitler's widely popular and demagogue style nailed down the political elite need for him (even though they admitted that did not like Hitler).
There are a multitude of reasons which saw Hitler's rise to power, and depending how you argue it, any of the above 3 can be the most important one. And all 3 of the factors I touched above helped Hitler in one way or another to rise to power as the Chancellor and later Fuhrer of Germany.
