Why didn't I get the symptom of public defecation?

Solution 1:

From the Plague Inc Wiki

Note that it seems to take a while for symptom combos to appear and take effect.

Unfortunately, there is no mention as to how long a symptom combo takes to become effective, if this duration of time varies for each combo, or even if this value is constant. Continued research yields little results as well.

I would say it is very likely that you killed everyone before the proper period of time expired for the symptom combo to become effective (or there may be other criteria, there is very little on how to achieve a symptom combo besides what symptoms are required to trigger it).

To have the best chance to get the desired symptom combo, I recommend evolving the required symptoms as early as possible (after infection of everyone is almost an after thought) while avoiding lethality. This will allow you to accrue evolution points while you're waiting so you can kill everyone soon after you get your desired combo.

Edit: As per the comment by @Nolonar and some research of my own, these combos seem to be rewarded after a random amount of time.