The excuses given are
- the situation is still very fluid
- unsure if they will become an oppressive dictatorship.
- continuity of use to avoid confusion
However
- When Communism fell, Lech Walesa and the Solidarity movement were immediately not refered to as 'rebels". Even though the situation was still fluid.
- When Ceauccescu administration fell, the new Revolutionary forces were rather fluid and a stable government was not present for two years. But they were not called "rebel forces" as soon as Ceauccescu was deposed.
- When the Iranian rebellion against the Shah succeeded, they were immediately called the Iranian Revolutionary government. The situation then too was fluid. And no legitimate elections have taken place.
- when Khmer Rouge fell, the international community refused to accept the legitimacy of the Heng Samrim and his fractured Salvation Front in an extremely fluid situation- they were immediately no longer called "rebel" forces.
So, why can't the news establishment stop calling the new governing authority "rebel forces", because technically the on-the-run former Syrian military and the Russian forces are the current "rebels". Also it is extremely unlikely that Assad would return to power to cause the revolutionary coalition of Syria to be rebels again.
Perhaps because no government has changed their terminology whilst the media is not neutral in some respects and prefer to defer to various governments' terminology preferences.
Perhaps the media does not want to be apparently seen as supporting a formerly, or still, jihadist authority but whilst they have had no issue terminologically "supporting" the Iranian jiadist rebel forces when the Shah was toppled.
What are the fine-tuning rules to decide whether a new Revolutionary authority is still the "rebel"?