The reason is that belief in "Some impossibilities are believable" guarantees its own truth.
Some impossibilities can be believed" may look contingent because the natural demonstration is prompted by my actual reply in a real-life debate.
The modal status of the commitment to believable impossibilities exposes a quasi-Godelian tension between deductive closure and consistency.
If it is certain that impossibilities can be believed, then the impossibilists already have their trousers half-way down.
Impossibilists grant that people often act as if they believe impossibilities.
But given impossibilism, believing someone believes impossibilities should be a special case of believing an impossibility.