The risk of compromise of the sitting president's account poses a much greater threat than does that of a former one. Also, the prominence of Twitter as a means for a head of state to communicate official policy came much further in prominence during the current president's term than before.
Also I'd guess that hijacking the sitting President's twitter would make the hack much worse in a legal sense. Plus now you've pissed off the president who will make especially sure your life becomes he'll.
Obama has a huge following (larger than Trump in fact) and I can think of a couple of tweets by that account that would have led to some serious trouble.
