Don,
Here is what Clark said in his Book of Abraham Symposium paper:



So according to this, Clark and Sperry were told "on several occasions" by the Church Historians that "perhaps" the Alphabet and Grammar had been lost. They were only granted access when they presented evidence that the documents were in fact in the Church's collection. It is possible that the Historians didn't know what they had, but it is also possible that Clark and Sperry were misled.
We also learn from Clark's account that he and Sperry were not given permission to speak publicly about the document even after they were granted access to it. As for Clark's claim that Sperry soon announced the discovery in print, Jerald and Sandra Tanner point out that this isn't exactly true. (Sorry about the bad quality of this scan.)

Sperry frames his comments as speculation or inference, and gives no indication that he had actually seen the document or that it actually existed. For example, he says, "It seems therefore quite probable that the alphabet was arranged very much as follows..." It seems in retrospect that he knew the layout because he had seen the alphabet with his own eyes, but his statement actually implies otherwise.
So, I don't think it's entirely correct that these documents were not suppressed. They may well have been released to a few scholars, but even that seems to have been done reluctantly and on condition of non-disclosure.
Peace,
-Chris