I've lived in Kerala for twenty years. Can attest to this being due to better healthcare. There's also very little underreporting of deaths happening (my sister's a doctor directly involved with the government effort).
It's a bit like Cuba, with the Communist past and healthcare/life-expectancy that stands out (compared to say, per-capita income which is average).
Kerala setup 23 Oxygen plants - most of them setup in the last year. It is now exporting oxygen to the other Covid-hit states. The vast majority of beds now have oxygen supply - even all medical colleges.
People actually planned and did things right in Kerala. No one did so at the capital.
But if the number of cases increases its very difficult to manage. Recent reports indicate Kerala might face shortage. Karnataka which also had enough oxygen can face a deficit in coming days if the cases keep up.
Someone who is from UP and lived in Kerala for few months, that state is world apart in terms of basic facilities. I mean computers in primary school (not to mention Linux)!!! In U.P., you won't find teachers teaching in most scholls.
One feature of the Kerala case, which does not get discussed much, is that the load is more or less evenly distributed throughout all the districts. This makes it easier to handle critical cases well.