There are a multitude of reasons for the aparent lack of publicity on the Spanish Flu.

One of the first reasons is that it started during WW1 in 1918, thus the state censorhip hid the true effects of the pandemic from the general public. Since the vast majority of countries with established media where at war in 1918, it would come as no surpise that the true effects of the pandemic where hiden from the people in order to maintain morale.

In countries that where not at war, such as Spain, the pandemic was reported to its full extent and created the ilusion the the country was particulary hard hit and even claimed to be source of the ilness. Thus the name "Spanish Flu" perfectly showcases how much people where kept in the dark.

The second reason is psychological and has to do with the notion of collective trauma. The pandemic came at the end of WW1, a collective trauma. It was thus timed with the conclusion of a horifying war and the years of despair asociated with it that folowed (1918-1920) which had society deal with the aftermath of the war: wounded soldiers coming back home, economical depresion and countless veterans dealing with PTSD, among these effects of the war, Spanish Flu got perceived as part of the aftermath, the fact that young people where most affected further cemented this subconscious corelation betwen the two events as young people had served in the military and where also a main vector of transmision.

Finally, it is speculated that the speed with which the pandemic would sweep a region meant that the media busy with war related news anyway had lityle time to cover it. Also comunities where being hit by waves of other diseases like typhoid fever and dyphteria which distracted the attention by making it look like just another epidemic.

So in conclusion, the main reasons for why Spanish Flu came to be "a forgoten pandemic" where a lack of information and the fact that it overlaped with the suffering during and after WW1 making it look like an extension of the former in collective memory.