Possibly because Gehenna refers to Hell and not Purgatory.
It does not. Hell does not appear in the Scriptures at all. It is a Scandinavian term.
What DOES appear in the Scriptures is Sheol/Hades - the underworld, where a chasm separates Gehenna from Gan Eden/Paradise.
Gehenna is a parched place of torment, where the unforgiven (because they were not forgiving) are imprisoned "until the last penny is paid". Not forever, for everybody is resurrected at the end of the world, and then proceeds into the City of God (not Heaven - nobody ever "goes to Heaven" - not before the end - Gan Eden/Paradise, where Abraham is found, is the Garden, not the sky) - and not after judgment either: the City of God comes to earth - OR they fail judgment and are thrown into the Lake of Fire for complete destruction - the Second Death. If you want to call the Lake of Fire "Hell" you may, but it is not the same thing as Gehenna. There's no "until" in the Lake of Fire after the end, there is the second death.
But now, before the end, there is Gehenna, and there is Paradise, and Gehenna is a place "until the last penny is paid", according to Jesus, and anyplace "until" is not forever.
That's what's actually IN the Scripture. Christian theology has departed significantly from that revealed information and inserted an eternal Hell before final judgment. Scripture has none. Nor is Scripture particularly clear that the Lake of Fire is eternal suffering for the human who fails judgment. For the Devil, yes, but for the human, it is the second death - whose exact nature is not described. It COULD be writhing forever in flames, but utter destruction of body and spirit - complete dissolution in the flames - is an equally valid reading.
Everything further in the Christian theology is adding to the text. You can do that if you want to. I will not. I won't even add the Scandinavian word "Hell", as it is superfluous and uneccessary. There is Sheol/Hades, composed of Gehenna and Paradise, and there is the City of God and the Lake of Fire. That is what is revealed. And after death, the prison of the unforgiving (and therefore unforgiven) sinner is described by Jesus as an "until" place, not a "forever" place.
I go with what HE said, and therefore I ignore the accreted tradition of Scandinavian "Hell" and "going to Heaven", neither of which are revealed, or present in Scripture at all. If one wants to call the Lake of Fire "Hell", that's fine, but the contours of Hell are not as the tradition describes, but as John described in his Revelation.
And Gehenna is a place "until the last penny is paid", which is not Scandinavian or Christian Hell either. That's why, just as the word "Hell" can be applied to the Lake of Fire, the word "Purgatory" can be applied to Gehenna. Since the latter seems to give people the hives, for the sake of conversation I am willing to just use "Gehenna" and leave off the Latin equivalent. But I'm not then willing to allow my interlocutor to import the Scandinavian word "Hell", with the concepts that come with it, unless he ties it to the Lake of Fire. Gehenna and the Lake of Fire are not the same place. They serve two different functions. No rich man will be calling across the black chasm out of the Lake of Fire, because that is after the end, after final judgment, and he'll be dead, burnt up in the flames, which was clearly not the case in Gehenna.
Gehenna fits Jesus' parables of the rich man and of the unforgiving servant, and also happens to fit Jewish belief. The Jews know that Gehenna - their "Hell" - is purgatorial, not Lake-of-Fire utter destruction.
But we've been over this ground before. What's the point? Christians have added a great deal of their own tradition on top of the actual Scriptures, just as the Jews did, and get stubborn about it.
For my part, I've said a couple of time what the Scriptures actually SAY, and I'm tired of Christian bickering - it's useless to me - so I am signing off this thread now. Believe as you would. If you actually pick up your Bible and carefully read through it all, cover to cover, focusing on what little is actually revealed about the afterlife in the Old Testament (you will find only references to Sheol until 1 Maccabbees, then you will find the efficacity of prayers for the dead). In the New Testament, in two parables of Jesus you find the function and operation of Gehenna, and in the Revelation you see The End, with the City of God descending, and the Lake of Fire and utter destruction after failed judgment.
It's all there. "Hell" - generic Christian Hell is not there. The purgatorial nature of Gehenna IS there, in Jesus parable of the unforgiving servant, and in the efficacity of the prayers for the dead in 1 Maccabbees.
If you don't want to accept that or believe it, don't. It's not skin off my nose. But it IS what is written there, not Scandinavian and Christian "Hell" and "going to Heaven". The only people who "go to Heaven" in the Scriptures are Enoch (maybe), Elijah and Jesus, in the sense that they are taken up by God into the sky. We are not taken up into the sky at death or at the end of the world. At death, we go into the underworld, which is composed of Gehenna, black chasm/abyss/Tartarus, and Paradise, where Abraham is with his bosom. At the end of the world the City comes OUT of Heaven, we don't go up INTO Heaven.
That's what the text SAYS.
But you can read it for yourself. I'm out. Not interested in bickering with Christians. I find it tiresome and useless. Believe what you want to believe. The Book will nevertheless continue to say what it actually SAYS, and unless you adopt what it actually SAYS, you'll be believing something different from what it says. Christian Tradition differs from what is written in the Book.