There was a big change in design philosophy. Early reactor designs were intended to prevent meltdown and had limited mitigation. More recent designs now include substantial mitigation as well as more robust prevention strategies.
E.g. The fukushima accident occurred because of a "common cause" failure of multiple safety critical systems - the redundant diesel generators. This failure led to a "cliff edge" cascading failure of numerous safety systems, effectively meaning that core melt was inevitable. (This is in addition to the incorrect site risk assessment, where an incorrect tsunami risk was used when assessing the suitability of the site for a nuclear power plant, and the additional failure to mitigate that risk when the tsunami risk was recognised in the 1980s).
Most modern reactor designs (the EPR excepted) do not class their diesel generators as "safety critical", because they are not necessary to place the plant in a safe state and initiate adequate reactor cooling. In addition, nuclear regulators (Japan excepted) around the world started carefully investigating "cliff edge" scenarios following the 9/11 attacks, to see if deliberate sabotage could result in disproportionate failure of safety features. In the US, the NRC started mandating that "safety critical" diesel generators be heavily hardened against beyond design-basis natural events and other methods of attack, even if not originally conceived at design stage; that UPS batteries be upgraded to provide up to 24 hours of safety, in order to allow emergency assistance to be called in, and/or that additional electrical power sources (e.g. gas turbines) be installed in fortified near-site (to mitigate against local site damage) installations.
A similar set of upgraded mitigations have also been in place for a while - hydrogen catalytic recombiners (these are basically catalytic converters similar to those in a car exhaust which react hydrogen and oxygen at a low temperature and low hydrogen concentration, well below the minimum ignition level. Heat generated from the recombination is used to cause natural circulation of air through the combiner to accelerate hydrogen removal and stir up the air to ensure that hydrogen cannot pool away from the recombiners) have been installed in-containment, and in buildings close to hydrogen vent pipes. In Fukushima, no hydrogen recombiners were used, instead the main containment building was inerted with nitrogen. As a result, hydrogen (and steam) built up in the containment pressurising the building. In order to reduce pressure to prevent rupture, the containment building was vented into the main reactor building, where the hydrogen mixed with air and later ignited. More modern designs vent directly outside through filters, or vent through hydrogen recombiners.
The other complicating issue is that at Fukushima unit 1, the reactor core appears to have completely melted through the reactor vessel into the containment building, severely contaminating the water in the containment building which was being used for cooling (and also leaked through minor damage to the containment). Again, modern designs try to mitigate this. The AP1000 design fills the bottom of the reactor vessel with low-melting point, sacrificial material into which molten core material will melt, resulting in dilution, prevention of re criticality, and spreading of the decay heat. Then by flooding the containment building and submerging the reactor with water, "melt through" is prevented because of combination of external cooling water and the diluted core material, as a result the containment building itself is not contaminated. The EPR instead, has a special chamber beneath the reactor intended to spread and retain molten core material, in such a way that it would not contaminate the containment building.