Design flaws. Many of them. 1. Lack of a containment vessel for the reactor core. All western reactors have these, this is a big deal. It's not a cure all but

Design flaws. Many of them. 1. Lack of a containment vessel for the reactor core. All western reactors have these, this is a big deal. It's not a cure all but it helps a lot. 2. High positive void coefficient for the reactor. This meant that as the reactor got hotter and/or was depleted of coolant the fission reaction rate went up. This is very super extremely mega bad. Most western reactors are designed with negative void coefficients so that they are more inherently passively safe. All RBMK reactors were retrofitted after Chernobyl to lower their void coefficient and tweak some other safety features to make them less insanely dangerous. 3. Graphite tips on control rods. This was the straw that broke the camel's back. Which took an "on the edge" reactor and tipped it beyond, which then exploded (in a literal nuclear explosion, most likely) and scattered debris across the entire world due to design flaw #1. Keep in mind that the Chernobyl and Three Mile Island events were fundamentally of a similar character, it was the difference in design between the two systems that led to one being a minor "incident" with almost negligible radiation release and one being a catastrophe with global implications.