The preference to choose YAML is due to the fact that YAML is many more times popular than TOML See https://trends.google.com/trends/explore?geo=US&q=yaml,toml
This makes a smaller learning curve for new developers in the team who come from various other tools into Netlify. From AWS to Ansible its YAML all the way in the DevOps space and, the TOML syntax is one more hurdle for new comers to cross to get into Netlify.
While TOML may be better in most cases, as a consumer who cares about faster on-boarding and a smaller learning curve, YAML is much better. And if the parser is good (can properly coerce values to the type it wants to understand), the proper indentation and removal of quotes makes the config easier to read.