Kayak customers can exclude Max 9 aircraft after cabin panel blowout on Alaska Airlines flight
Online travel agent allows customers to filter out Boeing 737 Max planes::Kayak customers can exclude Max 9 aircraft after cabin panel blowout on Alaska Airlines flight
Presumably it only shows models that actually fly the route you're trying to book.
There aren't many Max 9's in the world, it's a brand new model. Also in some countries the model is not allowed to fly with passengers on board right now.