It turned out that the Swine Flu version of H1N1 influenza had enough similarities to some H1N1 strains seen many years before, so people over a certain age tended to have a better immune response. The novel nature of a virus is what gives it pandemic potential, so when it turns out to not be so novel, the results dont resemble the terrible pandemics that really deserve the word pandemic. This is also the reason we didnt see a terrible death toll, and why that pandemic did not really tip people off about what a bad pandemic like the one we have now could be like. It was suggested that the response to that pandemic was an overreaction, and this did not eactly help encourage authorities to come up with more robust plans for future pandemics, or encourage authorities to act quickly with an abundance of caution.
Modelling of epidemic waves and peaks is heavily influenced by what percentage of the population are still susceptible. Things turn around when the pool of susceptible individuals that the virus can actually reach becomes too low to sustain continual growth to new heights.
We may also involve this in the question 'why were there 2 peaks with that timing?'.
To which the answer may very well be that it was a pandemic that largely affected young people, and school summer holidays broke chains of infection in a massive way, only to return when schools returned.
The picture I have described is likely to be incomplete and something of an oversimplification.