Because Mass attendance began to decline before the Vatican II reforms were made and in particular the 60’s.
People aren’t going to attend Mass if they do not connect with it.
How many people attend a Spanish Mass,(or you name the language) if they don’t understand Spanish?
Granted, there are some young people who do, connect with the Latin Mass. But they’re not the norm, but the exception.
BTW, I attended Mass on Monday, at the National Shrine of Divine Mercy, in Stockbridge Mass. Parts of the Mass were said in Latin. It sounded nice, but it did not have the meaning it would have had, if it were in English. I enjoy listening to Josh Groben sing in Italian, but I don’t understand the words he’s singing. So it is with Latin at Mass. Also, most people at the Mass, didn’t respond, some got distracted looking for the place in the hymnal, where the Latin responses were listed.
However, I’ve digressed. Not having a traditional Latin Mass, has nothing to do with the decline in attendance.
People who have faith, will attend Mass regardless of what language its in.
Jim