The number of researchers is not the only factor. In the 1940s, at the doctoral level, the obligation to learn a language other than the language a person was studying in was standard.
Around the end of the 1990s and at the turn of the 21st century, universities started eliminating that obligation to learn either a foreign language or the other Canadian official language, even for doctoral students in Canadian history, difficult as it may be to imagine studying the history of this country without being able to read one part of it.
The largest number of journals were started in the 1980s, but since then we have observed a decline. That phenomenon echoes globalization and the growing homogenization toward English.
Starting a new journal is not everything, however. There also have to be people to manage and fund it. If the readership is not very large, the business is much more difficult. In fact, several years ago there was even a pretty significant drop in funding for journals coming out of Quebec.
So a few factors, like globalization and a loss of multilingualism among the academic intelligentsia, contribute to explaining the present situation.