I think that the theory generally is that the occupants of the craft did not survive the crash. Consequently, there wasn't anyone to make contact with - merely some corpses and a lot of debris to analyze.
Apart from that, Harry Truman was a blabbermouth, and the idea he could have kept such a secret until his death 25 years later is not at all plausible.
The secret wasn't kept, at least initially: the Air Force announced that a crashed flying saucer had been recovered.
Occams Razor.
The government has said the debris was part of a balloon that was being used to test detection technology. It was made of radar deflecting metal. That sounds more plausible than a flying saucer.
There's argument about whether there was one crash or 3 crashes in a relatively short time that year in the same general area.
And, supposedly one ET did survive for some time from at least one of the crashes. Some claim a 2nd was alive when they first got to the site but expired shortly thereafter. But I may be mixing crashes up.
"I think that the theory generally is that the occupants of the craft did not survive the crash."
Or it could have been an unmanned, remote-controlled craft, which was another scenario considered by early researchers of some of the sightings elsewhere. As someone mentions elsewhere on the thread, the earliest reports did not mention any alien corpses, just the crashed object.