It is much more easily explained if you understand the fundamental weaknesses of the Weimar Constitution that used a proportional representation system for national elections that permitted the splintering of the vote amongst a multiplicity of minor parties and some major coalitions. These coalitions would shift and Chancellors would be forced from office. The effect of external factors placed before these coalitions some massive hurdles to succumb.

One must also acknowledge the political considerations of the majority party groups who consistently worked against the government. The Communist Party believed it should NOT work with the Social Democrats because the dialectic said that the exploitive democratic-capitalist stage had to be experienced before the communist revolution and thus worked for the rise of the NSDAP without clearly recognising the threat. Meanwhile, right-wing parties would also not cooperate with the Weimar Coalition because it frequently contained the Social Democrats.

The lack of experience with truly responsible democratic government is also an important factor as ministers did not clearly understand the idea of cabinet solidarity. They had not been made to understand this under the Second Reich system of government.

Interestingly enough, the modern German constitutions sets a base limit on votes to qualify for seats in the Reichstag = 5%