Cyrillic would make it accessible to 2/3rds of Slavic speakers, and would require that other interested parties learn just a few additional characters. It's really not that hard.
A good chunk of those 2/3rds speak English and can wield Latin script (and if not, they should).
Latin script has eaten the world; suggesting Cyrillic for any new language is lunacy.
The number of languages using Cyrillic is vastly smaller than Latin script, and are geographically clustered in areas that belonged to the Soviet Union.
Latin and English script share common ancestry and really are not that different. Likewise people don’t suggest to French and Germans to not use umlauts and accentuation, even though the characters are different.
For instance zx is really ž written digraphs.
Cyrillic, not to mention Glagolic, would make this kind of language less accessible to the world at large.