Because 'propaganda' got its pejorative connotation only after the Second World War.

Etymologically, the term 'propaganda' stems from Latin. The 17th century Congregatio de Propaganda Fide, Conregation for the Propagation of Faith, was founded by the Pope as a congregation dealing with coordination of Catholic missionary work in the new world. Propagation itself stemming from the latin propagare, to extend, to spread, to multiply horizontally. In time, the term propaganda established itself as the synonym for a 'propagation of ideas'.

That's all that it was meant, before WW2 and Joseph Goebbels, the 'Ministry of Propaganda' would be something we'd today probably call 'Ministry of Public Relations', or as /u/Praecipitoris said, 'Ministry of Information'. The pejorative connotation of propaganda that we know today only became established after WW2, largely owing to Joseph Goebbels' work by a sort of an extension of the Godwin's Law.

"Did you see what the Party said about that?" "Oh, that's just propaganda. It's like they learned those tricks from Goebbels himself."

To expand on the reply by /u/andriah_blashkovich , there are some previous replies that give more examples and information: "In history, was propaganda, as we know it, ever openly referred to as propaganda by the authorities who released it?" by /u/DV1312 , /u/NMW , and TarmacATK, and also "Did the Americans produce more propaganda in WWI than other countries? If so, why?" by NMW.

This is not to discourage discussion. More questions, data, and debate are welcome.

I realize that hindsight is 20/20 but how can someone whose title refers to the act of lying for the state possibly be believed?