Because he didn't go anywhere else. He could have joined Parler or Gab and continued drawing attention.
Just because you only pay attention to mainstream (D) media doesn't mean he's not on others.
edit: should have done my own research better, the profile reads:
>Reserved for the 45th President of the United States of America馃嚭馃嚫 This account is an uncensored Twitter archive and shares email statements sent by The Office of Donald J. Trump.
AFAIK Trump isn't on gab.