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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