It's inherently not private, and as an outdated format it can't quite support images (yes it can but big compression) and video (big compression) .

In some countries SMS is not free and might get quite expensive. This is one of the reasons WhatsApp became so popular in these places.

SMS are not free in the majority of countries. Group discussions are not easily conducted. Exchange of non-textual information (images, sound, video, etc.) requires something "more" (e.g. MMS).