Because it is only logical.
- Slovene is the first language, or, with another term, the mother tongue of most Slovenes.
- Slovene is the state language of the Republic of Slovenia.
- Language at universities (or anywhere else) is not only a means for information exchange. It also has a cultural, identity, symbolic, and emotional dimension.
- Most of the students and professors agree that it would make no sense at all if lectures for (mostly) Slovene listeners were in a foreign language (that is English).