A year long visa for Thailand means going through all sorts of hoops. Not so for Cambodia.
For Thailand it requires all sorts of letters, photos, police checks, health checks and the presentation of qualifications and resumes in quadruplicate. Every initial application must be at an embassy outside of Thailand while final approval requires lengthy waits at one of Thailand’s notoriously inefficient immigration offices. And even when the visa has been approved, Thai authorities still want a piece of you; an ‘alien’ must report to immigration every 90 days, just to say g’day and confirm their address.
Theoretically, it’s possible to pay much less than the amount the shops demand by going direct to the Cambodian authorities. But this is actively discouraged; they make you wait for between 4-6 weeks and who wants to be without their passport for that long?