[kzemek]

TFA does mention some points that address your "why"; the ones that most speak to me the most are:

> WordPress’s minimum requirements would be a simple PHP server, without the need for a separate database server.

> SQLite support enables lower hosting costs, decreases energy consumption, and lowers performance costs on lower-end servers.

This can be translated to lower operational overhead and faster loading times for small-database sites, and that equals money.

[unity1001]

> WordPress’s minimum requirements would be a simple PHP server, without the need for a separate database server

That doesn't help anything. The complications come from the plugins and themes that make the WP ecosystem. Not the WP core itself.

> SQLite support enables lower hosting costs, decreases energy consumption, and lowers performance costs on lower-end servers.

Thats too extreme a stretch. The flower shop owner in Oregon has bigger concerns than contributing to those with 0.0001%.

> This can be translated to lower operational overhead and faster loading times for small-database sites, and that equals money.

It doesn't, really. The reality of the software and infra costs in the industry is that they are already pretty low. And the meager improvement that will come with adoption of anything - leave aside SQLlite - just do not merit the effort that will go into doing it.

Majority of the web hosts already use reverse proxy caches that directly serve HTML instead of ever contacting the database. This applies to majority of the WP traffic including ecommerce traffic. If you are visiting any prominent WP web host as an anonymous user, you are very likely seeing a static HTML being served to you without db ever being contacted.

And in the case the db is contacted if the host does not have a reverse proxy cache or anything, it is only being contacted to get a minimum amount of information with the simplest of queries, and then again, a static page is being served from a caching plugin's cache.

Going further - if you are actually a logged in user, using a WP website without being served a cache and actual WP queries are happening, the chances are high that few of the queries that are happening can ever benefit from any db engine that is 'more optimized'. The majority will just use standard stuff, with MySQL servers that are already optimized for it.

So, all the glorious improvements that you imagine are not applicable in the real world. That's why nobody will lift a finger for it.