The phrase 'incomprehensible why' is correct and can be used in written English.
This phrase expresses confusion regarding a particular situation. For example, "It was incomprehensible why the school board decided to postpone the graduation ceremony."
And he'd said, with a look that suggested that her question was virtually incomprehensible, "Why not?
He also complained, "For me it is incomprehensible why we all had to wait so long for this decision".
As a collector of old books, I also find it incomprehensible why someone could fail to appreciate some dust jackets.
It is rather incomprehensible why the European court of justice would seek to put Europe at such a disadvantage".
And as the horrors multiply, it becomes even more incomprehensible why we should respond to any one of them (since we have not responded to the others).
But Labour MP John McDonnell plans to pursue the issue and says it is "incomprehensible" why the government is refusing to bring forward the regulations.
"It's incomprehensible to me why any manager would give anyone a two-month limit," Mr. Baer said.
It's a task that private actors can't accomplish effectively, since the players have a vested interest in keeping their products incomprehensible; that's why existing, private online insurance quote websites don't work.
It's an irrational project from an urban planning perspective and it's incomprehensible to me why a city which complains constantly about traffic congestion and wants to promote sustainable transit would allow such a thing.
"It's incomprehensible to me why Londoners can enjoy listening to a Manchester accent, a Liverpool accent, a north country accent, but physically recoil at a Brummie accent, to the extent that the image of Birmingham has been effectively banished from English feature film making.
