The spanish flu was much less deadly than bubonic plague on a case-by-case basis. It killed roughly 10-20% of infected people, whereas bubonic plague killed 40-60%. Just as an example, roughly half of Paris died from the plague, and up to 80% of people in some smaller Mediterranean communities died. The spanish flu had nowhere near this sort of impact on the areas it spread.

Where the Spanish Flu was more "successful" is in how it spread and in the amount of potential victims. Spanish Flu was found worldwide appearing within weeks on 3 different continents. By the second "wave" it had hit even relatively isolated pacific islands. Bubonic plague did not spread as easily, and the world was far less populated. Additionally, there was not a world war to spread the disease from soldiers to every corner of the globe.

Additionally, Spanish Flu is a virus (related to H1N1). This means it's difficult, if not impossible, to eliminate the actual virus. The most effective method available to treat this would be to treat the symptoms. The symptoms of the Spanish Flu were particularly hard to treat at the time (pneumonia and cytokine storm) and dangerous.

Eventually the Spanish Flu mutated (viruses can mutate quite quickly) to be less lethal after a couple years.

it may be worth x-posting this to /r/AskScience (Medicine category)

When comparing deaths from epidemics, it is important to look at percentages, not pure numbers. Yes, the Spanish Flu pandemic killed infected 500 million (and killed around a fifth of them), but that was less than 5% of the population of the world. The Black Death killed some hundred million in Europe, but that was more than a third of Europe's population.

Also, antibiotics would not help against a virus, but I am not particularly well-versed in medicine, much less early 20th-century medicine, so I don't know what made the Spanish Flu so virulent.