I can think of several points to answer your question.
-the developers are very talented and want to create an achievement for themselves so they create a new language where they, of course, would develop it to be different from the rest because they might have encountered several problems that other language does not have and hard to adjust.
-business purpose, developers can allow some free software yet add subscriptions to premium plan and more
Programming languages are just like businesses. Why open new shops when that type of shop has already been delivering good product and services? the answer is competition.