Explaining the cards, or doing tricks, back in the 18th-century salon.
In 1781, the French philosophe and Freemason Antoine Court de Gébelin published articles claiming to explain the ancient Egyptian origin of Tarot cards. These articles began public speculation about the occult nature of Tarot.
Court de Gébelin said he first recognized the truth of Tarot some years earlier while watching a group of ladies playing Tarot games in a French salon. He claimed he immediately recognized the ancient Egyptian allegory being told in the symbolism of the cards. Court de Gébelin became convinced that the ancient Egyptian ideas Tarot allegedly preserved only survived because people for centuries were unable to see beneath the surface of the playing cards and the games being played, to grasp what they were actually looking at: the ancient Egyptian Book of Thoth.
But, why if Tarot symbols were preservations of ancient secrets, would Court de Gébelin feel comfortable revealing the secrets to a bunch of card players at a Parisian salon, or later in greater detail in his 1781 published articles?
That is an interesting question, whose answer is rather complex. For one thing, Tarot was seized upon by Court de Gébelin as just one item in a long list of artifacts he alleged were veiling ancient secrets. The Tarot revelations would not have served the purpose of supporting his overall thesis, that apparently modern items like Tarot cards often point back to an ancient and wiser age, if he had kept quiet about it or only shared it with Freemasons.
Also, in French salon culture, the philosophe, such as Court de Gébelin, was there to instruct and enlighten the other salon attendees. In fact, that was a big part of the attraction of the salon—you just might learn something or get to hang out with people learning something from the great minds of the day.
Having a sudden revelation concerning the ancient origin of Tarot allowed Court de Gébelin to show off his erudition, and to perform his public function at the salon. There was a great difference in atmosphere between an 18th-century salon, where people were openly attempting to share information with the literate public, and late-19th-century secret order gatherings, where secrecy was the greatest concern. The secret orders, which to varying degrees still accepted Tarot as the Book of Thoth, and as a central secret of their teachings, proceeded to veil and mystify what Court de Gébelin had openly taught.
At the same time, occultists have always openly discussed Tarot, always alleging to fix it and make it wholly legible to people attempting to read it or read with it. But at the heart of this seeming beneficence is an essential dissembling, necessary because Tarot became mired in the struggle to prove the authenticity of claims of one occult group and its masters against another.
If you revealed all your secrets like you were an 18th-century philosophe, you wouldn't have anything left to peddle or reveal back at your secret order.
So, a concern to establish authority and keep the cash-paying members coming in is the primary root for the notion that Tarot is mysterious, containing secrets only the enlightened can understand.