He spoke in the everyday language so everyone could understand him without needing priests to explain.
From the Discourses
Passages where Osho speaks to this question — each links to the complete discourse.
Osho, Kastur Bhai has asked: Why did Mahavira speak in Prakrit and not in Sanskrit?
And I take one basic reason to be that Mahavira spoke in the language of the people. He likely had very little contact with pundits. It may be that thousands of pundits remained unfamiliar with what he was saying. For pundits have their own bourgeois snobbery, an aristocratic attitude; they are not common folk. They neither speak nor think in the language of the common. They are the extraordinary, the chosen few, and everything in their chosen world is apart from the ordinary. The common people are outside the mansion, outside the temple. Sometimes, out of pity, out of grace, they tell the common folk something—but the deep and serious discussion goes on inside the temple, where the illiterate and the ordinary are forbidden to enter. One of the greatest revolutions of Mahavira and Buddha is also this: they brought religion straight into the marketplace, right into the heart of…
Question: OSHO, WHAT PLACE HAS MYSTICISM IN YOUR RELIGION? Mahavira never spoke in Sanskrit, Gautam Buddha never spoke in Sanskrit -- for the simple reason that they were trying to defy the priesthood. They spoke in the language of the people. They were condemned by the priesthood: "This is not the right way. You should speak in Sanskrit. And both of you are perfectly well educated " -- both were sons of great kings -- " you know Sanskrit, so why do you speak ordinary people's languages?" They said, "For a certain reason: we want people to know that this mystifying has to be exposed. There is nothing in your scriptures, but because they are in a language which nobody understands, it is left to the people's imagination." Even the priest may not understand what he is reciting because Sanskrit has to be learned by memorizing, not by understanding.
Osho, what kind of Indian guru are you? We’ve heard you don’t even know Sanskrit!
English, therefore, is the most alive language on earth today--every year it digests eight thousand new words. No other language has such digestive power. It digests everything. You’ll be surprised: English even digested the Sanskrit word “pandit.” “Pandit” is used in English; the meaning is the same as in Hindi--a pompous pundit, hollow, stuffed with cow-dung. Digestive power is the mark of a living being. They can’t even digest “railgadi,” though it has caught on! Millions use it. The whole country understands “railgadi.” Neither Tamil nor Marathi nor Gujarati nor Punjabi nor Hindi has any difficulty with it. But he coined a word, and it didn’t fly. He tried very hard to make it fly--“lohpathgamini” (iron-path-goer)! It just doesn’t sit right; it sounds stupid. You’re going to the station, someone asks, “Where are you going?” You say, “I’m going to catch the lohpathgamini!” He’ll think your wife has run away…
Osho, yesterday you said that jealousy is included in respect. I have immense respect for you, but the jealousy inherent in it keeps poisoning it, and I feel guilt and pain. Does reverence transcend this poison-laced respect?
It needs a little explaining—it's a delicate point. Whenever you respect someone, you do so because you see in that person something you do not have. You respect because you glimpse in the other something you would also like to possess. A beggar respects an emperor because he, too, longs to be an emperor. So on the one hand he respects, and inside he also envies. Because he is not yet an emperor but wants to be. You have attained what he wants to attain. He respects you as skillful, successful: “I stand far back in the line; you have gone ahead to where I should have been.” So you are powerful, clever, intelligent, strong—he respects you. But inside a fire of jealousy also burns—if he gets the chance, he would like to be in your place and push you aside. And if the beggar gets that chance, he will…
Beloved Osho, mahavira was the twenty-fourth tirthankara of the jaina religion. Did the jaina religion start with the very first tirthankara or with mahavira? And what is the meaning of the word `jaina'?
And they became really rich, so rich that even kings had to borrow money from them. Nobody was a beggar, nobody was uneducated. And because they were such a small minority, attacked by all kinds of philosophies, they had to protect themselves; they sharpened their intellect. They created better arguments than anybody else, because for others it was a luxury to argue, but for Jainas it was a question of life and death. They had to win the argument; otherwise they were finished. So they have developed logical systems, great philosophies, which should be made available to the whole world. But the world cares only about numbers, and they are a non-converting religion, so the question of creating great numbers like the Catholics is not possible. They are non-converting because, according to them -- and I accept the idea -- the very effort to convert somebody is ugly. You can…