Personal profit, zealotry, career seeking, incompetence, design flaws, political agenda. This also includes the design phase. The RBMK reactor was an irresponsible design from the onset, even without the unknowns.
Graphite moderated reactors are prone to graphite cracking, as also evidenced by UK's AGR reactor fleet. Maybe pebble bed reactors are safer, because new pebbles are continuosly fed in and the spent ones are extracted for reprocessing. We'll se how the HTR-10 and the HTR-PM fare.
The handling, as in, the reaction once the top officials actually understood the magnitude of the situation, was nothing short of spectacular.
No expence was spared cleaning up the mess, removing top layer of soil at a massive scale and enclosing the failed reactor in sarcofagus. This expence and reputation damage contributed considerably to bringing the end of USSR.
You've got to keep in mind how little was known about lethality and handling of radiation back then, compared to today. In fact good chunk of today's knowledge comes from Chernobyl.