Why do you think so many people, especially elderly residents, were so reluctant to evacuate their homes, even though they were receiving warnings about imminent flooding and landslides in Kurashiki, Hiroshima and other hard-hit areas?

It is human psychology. They've been through similar before and weathered it, so why should anything be different this time?

Or

Most people don't think these disasters will happen to them.

This happens all over the world where predictable natural disasters happen and where a history of disasters lead people to believe that the 100 yr version that is really bad won't happen this time.

Despite repeated disasters, the public continues to turn a blind eye to the risks. Developers construct buildings from unsafe materials and in hazardous locations, rather than investing slightly more to avert dangerous consequences.

Ref: https://www.livescience.com/38036-why-people-unprepared-for-natural-disasters.html

Similar to what @theFu said

I think having experienced past floods, typhoons and earthquakes created a somewhat complacent thinking, futhermore the officials in charge had the same thinking so alerts were either late or too conversative.

The problem with floods is that when you change your mind too late you have no way out anymore.

They didn't think it would be as bad as it was, also many older people just can't up and leave without assistance.

The assumption is that they were refusing to, rather than being unable to, unsure where to go and how to get there. Was that the case?

I know that disabled people are often not helped at all at these facilities, and some prefer to stay home for that reason.

The whole experience is camping out like a refugee is highly unpleasant even for people without disabilities. And quite frankly, the odds of perishing in any given situation are quite low. Even in this recent, once in 100 years scenario, there were a couple hundred deaths spread out among millions who were affected.

Statistically, it's not all that dangerous to stay home unless your house is situated on a particularly vulnerable site. In a worst case scenario in Japan, some people will die, but the overwhelming majority will not. That's why people take the risk.

I'd say a lot of it comes down to simple stubbornness. Some old folks here are so set in their ways they don't respond to disaster warnings. Hopefully one day they'll learn

Perhaps some old folk don't want to leave all they know behind, they don't want to inconvenience others and maybe they aren't aware of evacuations going on.

We'll all be getting on years soon enough, no need to denigrate the elders.

If you are told to get out of your house for fear of floods, you get out! If you don't, you die. It was similar in the 2011 tsunami. Many people did not evacuate and they paid with their lives. Also, it is quite plausible that a small percentage of the elderly victims had no form of transport.

There was some talk about elderly not receiving the warnings due to not having smart phones, watching TV, listening to the radio, etc. However, the local councils can drive around in those annoyingly loud speaker buses during their campaigns. These same politicians should have been driving around getting people out of their houses.

Probably they feel much safer inside the house than going out to evacuate.