Glagolitic very quickly got pushed out by what were essentially Greek letters. If you look at Bulgarian and Byzantine manuscripts from the time, they are almost impossible to tell apart, unless you know the languages.
The reason for that is pretty obvious if you look at the Glagolitic letters themselves: they are horrible UX. You need a lot more strokes than for something like Greek or Latin to record the same information. Because Glagolitic was contrived and not polished with use over the centuries, there was very little reason to use it over Greek.
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"She relaxed in the chair under a tree's shade"
"Шe relaxed in фe чair under a tree's шade"
None of the interesting bits of Cyrillic invention are covered, like how the original Slavic script was Glagolitic as the sibling mentioned, and only evolved into modern Cyrillic much later. Or how there was no lowercase until a few centuries ago, especially with the reform of Peter the Great.
With Slavic people, it's also worth noting that "Slav" actually means "word" or "letter" (of an alphabet), so legibility was part of the identity. In contrast, most Slavic people call Germans a variation of "Nemci", or mutes (those who cannot speak) — notably, most except Russians who call them Germans. Again, likely to distance themselves from the negative connotation with their aspiring historical partners.
In reality, at the time, it was the Eastern Christian church that was more liberal than Rome. Rome insisted every local church make services in Latin, and didn't translate it in the local language.
The Eastern church instead, had the bible in Greek, but allowed to translate it in local languages and make services in them. Initially, those translations were made with Greek letters, which weren't fully reflecting the phonology of Slavic and other languages, so they were extended, which produced Cyrillic.
As I understand, the same way Coptic script in Egypt, and Ge'ez in Ethiopia were made, thanks to Eastern Christian church allowing this.
p.s. Saint Cyril, in fact, invented the Glagolitic script. Cyrillic was named after him, and initially "Cyrillic" alphabet was mostly Greek, plus some characters from Glagolitic, like Ⱎ, ⱍ and ⱑ.
On the political aspect Russia has always hated the fact that small Bulgaria gave them their alphabet/culture and has used it's influence to bitch, moan and subjugate ever since. Most recent rage bait is with bullshit like saying that it's actually from (the country now known as) North Macedonia.
* Sviatoslav was not a local ruler - he ruled Kievan Rus' 1500km north-east and he remained a pagan until his death, even if his mother had converted to Christianity.
* Sviatoslav was born nearly 60 years after both Cyril and Methodius had died.
* In 890 Boris was no longer in power but his firs son, who coincidentally tried to reverse the Christianity conversion and was kicked off the throne a few years later.
* " Just after the invasion of Ukraine in July 2021" check the date.
Just what type of slop this one is? It was not "just after the invasion", it was ~7 months before the invasion. At least if I understand correctly that the start of Russo-Ukrainian war is called "invasion" here.
