Why didn't the totalitarian parties in the 20th century use the "big tent" strategy?

Question

I see that ‘big tent’ or ‘catch-all’ parties are quite popular in modern politics. However, the autocrats of the past took the opposite view:

Before we can unite, and in order that we may unite, we must first of all draw firm and definite lines of demarcation. <...> we do not intend to make our publication a mere storehouse of various views. <...> A party becomes stronger by purging itself. (Lenin)

It is quite erroneous to believe that the strength of a movement must increase if it be combined with other movements of a similar kind. <...> Through the formation of a working coalition associations which are weak in themselves can never be made strong, whereas it can and does happen not infrequently that a strong association loses its strength by joining in a coalition with weaker ones. (Hitler)

Why were Lenin and Hitler opposed to the unification strategy that gives the majority to many contemporary European parties?

update: Why, for example, CDU/CSU and SPD in Germany don't split into separate parties that would argue with each other?

Answer 1

Minor and less often substantive ideological divergences were often the basis for those purges, often coupled with treason charges etc.

If you agree 100% with the leader on everything (often painted as inerrant in personality cults), you can't possibly challenge them on anything, so [you] are not a political threat, on a personal level. Which is usually the point of authorianism: eliminating as much opposition as possible, preferably all of it.

Only in extremely limited circumstances, like when a leader is extremely and visibly sick, does a line like "I 100% agree with you on everything, but I think I should lead the country instead" have any chance of working.

Answer 2

Lenin and Hitler talked about different things here.

Leninist view
Lenin is referring to internationalism, the concept from the Communist philosophy, which sees the historical evolution as a fight between social classes (peasants, proletariat, borgeoisie), defined by their economic conditions, regardless of their national origin, race, sex, etc. This struggle eventually culminates in revolutions resulting in a more progressove social order (pimitive society->slavery->feodalism->capitalism->communism).

In this light, a relatively minor point of disagreement between Lenin and some of his party camarades (notably Trotsky) is whether the communist revolution occurs simultaneously in the whole world, or whether revolutions happen independently in different countries, which will then unite in a single classless society. Lenin correctly judged that while Russia was in a position where the communists could take power, the world was not ready for the revolution. Trotsky and others disagreed - it is worth noting that the Great October Socialist Revolution occurred during World War 1, which communists interpreted as an imperialist war contrary to the interests of working people; notably, the massive fraternisation between French, German and British troops gave reasons to believe that the soldiers would eventually unite against their real enemies (i.e., the capitalist governments). Trotsky famously tried to use the separate negotiations in Brest-Litovsk to gain the time for the German soldiers and workers to overthrow their government, whereas Lenin insisted on actually make a peace treaty with Germany for the reasons of the domestic politics (the war was highly unpopular and soldiers could be used as a force against the Communist domestic opponents).

Nazi view
Nazi view was in essence nationalistic, even if containing many socialist elements when confined to a single nation (it is necessary to note that most socialist tilts were abundoned after the Nazis came to power and made alliance with the big industrialists; this eventually led to the bloody purge of the socialist elements in the Nazi part, even though these were instrumental to the Nazi rise to power). For Hitler uniting with others meant making alliances with similarly nationalist movements in other countries - notably the Italian Fascists, Francoists in Spain, Petain followers in France, etc. Nazis didn't mean merging different nationalist movements into a single one, but simply considered them as ideologically and racially equal.