Old Czech (Bohemian) or Moravian

[Stefan Wirtz_2]

[..] What could have been the reason to replace Glagolitic, which was a fairly commonly used Slavic script?[..]

Speed?

Glagolitic has a lot of most-complicated symbols, very uneasy to write.
"Voynichese" symbols are made quick-and-easy, whatever the rest of this idea was.