KFX is the modern kindle format, AZW meanwhile is heavily PDF-based. KFX was designed ground-up by Amazon, supports every modern feature they could think of, and presumably couldn't be backported to 2013 and earlier Kindles; AZW meanwhile was basically a wrapper around a subset of PDF. KFX is a complete redo, notable enough it's what "Enhanced Typesetting" on every Kindle product page means, not a small DRM upgrade.
By doing this, all authors will soon receive guarantees that they will have the full KFX feature set when designing eBooks, and won't break AZW by accident. Trying to point this out though to the "it's about DRM" or "it's about obsolescence" crowd will get you downvoted to oblivion before the truth is even considered (speaking from experience, -4 when I dared suggest legitimate reasons exist) and is a prime example of echo chambers and deeply ingrained bias on this forum.