Because those are CIC's rules. Some private colleges do meet the requirements for the PGWP - but many don't. The best decision you can make is to avoid private colleges and attend a public college instead.
Generally speaking, private colleges are a bad choice if you want to have a future in Canada. Their education quality is very poor (comparing to public colleges). Private colleges also have bad reputations with employers and it's much harder to get a good job if you've graduated from a private college (because employers know they are bad schools).
