The reason behind the change was mentioned at the very first line of the announcement thread from theymos.
Due to excessive garbage-posting
It's true that there are lots of ranked people who are posting garbage too.
But if we compare with the Jr member, it's very much low. For your info, as far as I can remember, there were only 1.2 million member back in August 2017 while it has been increased hugely right now. So, most of them are still Jr member, not a higher ranked member.
Therefore, it's logical to put restriction on Newbies/previous Jr member.