I don't think there are many airlines flying this route as Sihanoukville airport only reopened about 6 months ago. In that time one of the flights crashed, killing everyone onboard. Probably best to take the bus! The only airline flying the route is
PMTair. Runway is being extended so I imagine more airlines will fly there within a few years.
When I was in Cambodia there was no direct bus so I had to transit in Phnom Penh and jump on the next bus to Siem Reap. Took around 9 hours in total. Probably a direct bus now though.