Because if not then all the rank will back to newbie so it's given for free, don't forget Legendary members gets 1000 merit don't know how much sMerit they got from system.
I think you already know that so I'm not going to tell you anymore that Legendary gets 1000 merits when the merit system is implemented and also has 100 to 150 smerits in it.
And what I remember is that only Jr.Member rank dropped to Newbie a month after there were new rules so that those who had been in Jr.Member had to try to get one merit in order to be in Jr.Member Rank again, while for Member Rank it was automatic has 10 Merit units at the time the merit system was implemented.