For anyone unfamiliar, Khmer is phonetically written, with (from an English speaking perspective) complicated rules for character placement thats arent always 'linear', eg some characters go above or below the previous or next character. The phonetic order is confusing. As mentioned in the article, there are a lot of characters, and no spaces between words. There are also at least two competing systems/programs for typing these characters on a standard keyboard.

There is no real standard romanisation similar to Pinyin for Chinese. The UN tried creating one in the 90s, and there are a couple of homegrown alternatives, but as mentioned in the article, most people just kind of make up the spelling and hope the recipient can figure it out. Until quite recently lot of people still had 'dumb' phones sending character limited sms, so spelling has been 'creative' (I stopped carrying a dumb phone as a daily driver probably around 2017-18, so really quite recent).

Basically as the article says, its a huge pain in the ass to type in Khmer, especially on a phone. If that crack team of overpaid consultants had jumped on the phone to anyone in Cambodia, they could have explained why voice messaging is so prevalent.