After doing plumbing professionally for 10 years I’ve come to the conclusion that the main reason is difficulty of insulating PEX once it’s installed. Plumbers don’t generally do insulating in commercial jobs. There’s inherently no such differences in quality that would make PEX unsuitable for most applications. Still we seem to prefer either composite pipe for water or steel for heating. PEX mainly when it’s installed somewhere you can’t check for leaks later bexause with PEX you can do it with minimal to no joints. Underfloor heating for example.