I have found that a company can be uncomfortable in being a reference for a few different reasons. These are not exhaustive but are some of the reasons.
1.) Company policy (especially in large corporations)
2.) They don't want hundreds of other companies calling them and wasting their time
3.) They don''t want to advertise what tools they are using to accomplish their goals
4.) They aren't a big enough fan of the product/service in question
The main reason is economics.
Almost all companies have descriptions of their products and services along with trademarks, copyrights etc. Any company providing "a reference" would need to follow these guidelines, in almost all cases the only way to avoid infringement is to talk to the company and agree what can and cannot be said. At this point the "reference" is basically something that the company you want to know about has written ie useless.
Let's say company A hates company B because of some dispute, asking company A for a reference could put them in some hot water if they make any claims that company B takes umbrage with.
Let's say company A loves company B but when providing evidence of such breaches some trademark or copyright or something else and company B asks them to retract it etc.
Now, if you offer a billion dollars to company A to find out about company B that might be different but it you're asking and there's no upside why bother!