The main reason is the Treaty of Versailles.
No. Why? Because remove the ToV, WWII as it inevitably played out as it did is still entirely possible. It is a minor contributor at best.
I believe that if you want to answer this question, you need to try and find a factor that is essential for thigns to play out. WWI isn't so easy, but for WWII, it is.
You want to butterfly away WWII as per OTL, remove one man.
Hitler.
The math is simple, you remove him, the primary cause of the Second World War as per OTL is gone. He is essential to Nazism and without him, Germany would have taken a very different road, but you can't say that war on a scale that OTL WWII took place was the only inevitability.
Yes the ToV was a selling point for the Nazi's but, it was one among many. If we removed the ToV, we don't don't butterfly away the war, because the other compounding economic hardships are still there and it is still entirely possible for Hitler and his cronies to take control.
If you are going to blame the ToV, you may as well blame Otto Von Bismark for unifying Germany in the first place.