They turned off every security check to test what would happen.
Doesn't seem very smart to me.
If I had to pick one reason, I would blame reactor design. The RBMK reactor had what’s known as a positive void coefficient. When voids (boiling water pockets) formed, these reduced the reactor’s ability to moderate neutrons, thus accelerating the reaction rate. This makes the reactor fundamentally unsafe, and guaranteed that something was going to go wrong.
This is a design flaw that has thankfully been fixed in other reactors, including all other RBMK reactors. The flaw was known to the Soviet’s at the time, but the RBMK was designed to produce nuclear weapons, so they swept the flaws under the rug.
Actually, what the loss of water did was reduce capture of neutrons on hydrogen. The moderator, graphite, remained in place and was more effective.
But human error happens and has to be in your risk calculation. Just like natural disasters happen (Fukushima was insured, just not against earthquakes, tsunamis and the like)
Edit: for context, "unusually expensive" means we hope to get the whole thing fixed within the 21st century.
Now that we actually start decomissioning them in Germany it turns out that decomissioning nuclear plants is more expensive than constructing them. Everything has to be measured for radioactivity, with a significant portion of the structure turning into nuclear waste due to being exposed to radioactivity for decades.
Insuring nuclear plants at a fair rate would also be really expensive. While serious accidents are rare they are exceedingly costly. The Fukushima disaster cost somewhere around $100-200 billion. Ukraine still spends 5-7% of their national budget on Chernobyl, 30 years after the disaster. Somewhat strangely most of that financial risk is taken by the government instead of the plant operator.