Because people like dark humour.
I have a rather twisted and dark sense of humour - some would even call it macabre actually - and I can even appreciate some jokes about this very subject, but it never prevented me from using common sense so I can understand that trivializing Hitler and the death of millions by including him in a colouring book is actually just irresponsible and stupid.
But for anyone completely desensitized who lacks common sense, indeed, sense of humour is a most convenient excuse.
If someday people worry about showing empathy a tenth as much as they worry about displaying their so-called sense of humour, we'll probably be able to laugh at
everything. I wouldn't hold my breath though.
But that's just my 2 cents.
EDIT - FWIW, this was not meant as a personal attack against you. It's my reaction to general observations which seemed appropriate in the context of this discussion.