An ordinary text message might well have sufficed in this instance from an end user point of view, sure. But there isn't an existing system to manage that.

You'd need to get the mobile operators involved to build the list of recipients and then to do the actual work of pushing the message out. Normal SMS is point to point, so a message needs to be sent to each subscriber individually which is a huge job at population-level scale.

It makes more sense to use the existing cell broadcast alert infrastructure which has been planned for exactly this type of use case and has been thoroughly tested.

A more reasonable complaint might be that it wasn't geofenced to only the drought-hit areas, but that would only have excluded something like a quarter of the population.