Pardon the breach of rules for a moment: IT'S EASIER TO READ THIS THAN it's easier to read this.
If readability of lower-case was the issue we would write all variable and function names in upper case, even in languages that have lower-case keywords.
Secondly, upper-casing some parts is not about global readability; it's about contrasting code on monochrome displays.
The PARTS IN CAPITALS are the parts the language (compiler or whatever) defines and must have spelled exactly that way. The parts in lowercase are the names, symbols etc. that the programmer defines rather than the compiler or OS.
Personally that is how I learned and even now, some 40 years later, I find that C-style everything in lower case is MUCH less readable for me than mixed CAPS and minuscule.
I'm just wondering what the utility is today for people who have questioned it but decided to go with uppercase for parts of the code anyway.
As someone else said, if one is prone to confusing keywords with identifiers, syntax highlighting has existed for many years now, and solves that problem MORE elegantly than INTERSPERSING a few SHOUTED words in the CODE.
I dislike the PEP8 style for python, but I use it anyways, mostly.
Lowercase letters requiring descenders (g, j, p, q, y) might be relevant.
When we read text, we don't read letters, we read words. So familiar word shapes help reading text more quickly. Lowercase letters have more distinct shapes (due to ascenders and descenders) so they make for more distinct word shapes. Uppercase word shapes are not only less familiar (because most text you've read is not all-caps) but also less distinct because they lack descenders and all have roughly the same height.
So in other words, writing keywords as in SQL in uppercase can be more readable because a) it indicates it's that language (which can be helpful in mixed-language source code), b) the keywords usually being written in uppercase make them easier to recognize without having to be familiar with the words from other contexts (this might actually be an advantage for people who don't read as much English text) and c) this sets keywords apart from variable and attribute names without syntax highlighting.
For filenames and such there is no such advantage which is why you only occasionally see all-caps used for special files (e.g. LOCK, VERSION, README) where the uppercase text is intended to stand out from other file names and usually follows a naming convention that makes the word shape common enough to be easy to recognize.
