star wars - Why would the Force become so mysterious and unrecognized after only 20 years?

Basically because while the Jedi order was well known and famous, most people never met a Jedi or saw them ever do anything with the force. It's akin to the bible and the old testament where, since no one has ever seen God do any of the old testament type mircales, most people have relegated it all to nothing but myth, stories or compelte fabrications.
So it's the same with the Jedi, the Jedi order and the force.

Same is also true for the Shaolin, chi and chi-gung, because today there are so few practitioners that can really use their chi, have the kinds of skills, power and abilities that you would see in old school kung-fu movies, and those that can do such things do not advertise or show off... most think that they never existed and that they were mere exaggerations or stories, myths,..not real.

Much of what goes on in the Star Wars universe is taken from both the history of the Shaolin monks and the biblical prophets, so that is why I use those parralells.

The Jedi were also considered to be traitors to the republic and the Palpatine and Vader stopped at nothing to wipe out as many records of their existence as possible. Also, Palpatine was waging a massive propaganda campaign against the Jedi during the Clone Wars. After order 66, anyone even remotely having anything to do with the Jedi were labeled as traitors and jailed or killed, so this scared people into simply forgetting anything they kenw about the Jedi, do not discuss them, read about them, etc. Those that were older and possibly had seen or met a Jedi would know better, but the vast majority would be taught via propaganda, state controlled media, edited text books and the like, to beleive that they were nothing more than a myth and legend.

Even on Obra-Skai, which was the largest repository of knowledge in the galaxy, most of the records of the Jedi, along with their knowledge, training techniques and more were wiped out. Luke was essentially a walking library of knowledge of the Jedi techniques, the only one known to exist at the time.

Later on, Luke eventually found the Chunthor', which was a mobile Jedi training ship that he found crashed on Dathomir. He was eventually able to recover training disks that the witches of Dathomir had kept since Yoda went there over 300 years prior. These are things that Luke would have learned from Yoda had he not disobeyed Yoda and Obi-wan and left for Bespin in an attempt to save Han and Leia.

So there was definitely a massive concerted effort to destroy all memory of the Jedi from the entire galaxy, no doubt greatly contributing to most people beliving that they never really existed.