Have there been any cases of cerebral hypoxia-induced executions in US or elsewhere in the world?
The answer to that is in the very article you linked to:
There are no reports of nitrogen gas ever being used to execute humans...
If there have been no such executions, why is this the case?
Again, answered in that same article (same paragraph):
critics say that one concern is that the method is untested. Some states even ban its use to put animals to sleep.
Granted, there is a counterpoint:
But supporters of Oklahoma’s plan argue that nitrogen-induced hypoxia – or a lack of oxygen in the blood – is a humane execution method. “The process is fast and painless,” said Christian, a former Oklahoma Highway Patrol trooper who wrote the bill. “It’s foolproof.”
I don't know Christian's credentials in regarded to human biology, but my guess is that the reason it's not in widespread use is simply because the 'foolproof' claim seems tenuous, at best, at this time given the fact that it's never been actually used on humans.