We attribute AirPods' dramatic growth (14–16mn, 26–28mn, 50–55mn, 70–80mn and 100–110mn units in 2017, 2018, 2019, 2020, and 2021, respectively) to 1) the upgraded model with wireless charging support to benefit shipments to launch in 1Q19, 2) the all-new design model to boost replacement demand to launch in 1Q20, 3) an innovative user experience, and 4) high integration with iOS and Mac products. In summary, AirPods is Apple's most popular accessory ever, and it currently has the best growth momentum among Apple products.
There are "great potential growth opportunities" for AirPods since less than 5% of the over one billion iPhone users currently own a pair.
It's believed that Apple may be waiting on its much delayed AirPower charging mat to launch the new AirPods. You can follow iClarified on Twitter, Facebook, Google+, or RSS for updates on both devices.