English has two be’s, equative and predicative, and both of them seem uniformly to be bad with it-clefts. Compare:
Noun phrase:
Susan is president / a president / the president.
?? It’s (a/the) president that Susan is.
Adjective:
Susan is horsy.
?? It’s horsy that Susan is.
Prepositional phrase:
Susan is under the table.
?? It’s under the table that Susan is.
And for something truly appalling, a wh-phrase:
Susan is who you should ask.
?? It’s who you should ask that Susan is.
This seems to be a fact about the interaction of it-clefts with be, rather than just be by itself though, as many of these sentences are fine with wh-clefts; for instance:
What Susan is is the president.
What Susan is is horsy.
Where Susan is is under the table.
My guess is some clever linguist specializing in the semantics of it-clefts will have to unravel this nice catch of a conundrum.