Complex sentence: Earlier in Russia "JIUKEN" ("ЙІУКЕН") layout was the main layout, but then it was replaced by JCUKEN, because of the Russian reform of alphabet (removing of Ѣ, Ѳ and І letters).
Split into simpler sentences: Earlier in Russia "JIUKEN" ("ЙІУКЕН") layout was the main layout. But then it was replaced by JCUKEN, because of the Russian reform of alphabet (removing of Ѣ, Ѳ and І letters).