Higher education in the US is largely self-regulated via the accreditation process. The government has minimal oversight of that self-regulation, mostly through the GI bill. Self-regulation doesn't mean they collude in a bad way (though there are suspicions that the rising cost of tuition is a collusion problem). Self-regulation is a form of collusion that, when done right, is good for producers and consumers - LEED or certain organic food certifications or academic accreditation are easy ways for consumers to know what they are getting and for producers to limit the negative price and quality pressure of "lemons" (bad products) on the marketplace. And it achieves this effect without heavy handed bans that limit consumer choice.

whether the regulator has been paid off or not. seriously though,

barriers to entry, the number of competing firms, anti competitive behavior etc.

Low barriers to entry mean that if existing firms collude to artificially raise prices a new competitor may easily enter the market to undercut them.

A larger number of firms makes collusion less likely (see OPEC for example). Their attempts to collude are often met with one member state undercutting the agreed upon price and getting most of the business.

Other factors are also in play such as whether the industry is most efficiently served by a single entity as in a "natural monopoly"