Until someone does a representative survey with members of the Vintners, we won't really know why the majority of pub owners aren't stocking craft beer and we will all be speculating idly.
Which I'm happy to do, so here's some of my thoughts.
For one, some pub owners I have had the misfortune of talking to over the years, seem to know very little about the product on which their livelihood is based. Conversations about beer styles with them is akin to talking to a butcher who honestly couldn't tell the difference between pork and steak and who only eats sausages anyways. And not those trendy craft ones either
Often owners have inherited their trade. Some people are bone idle and are happy with just keeping things ticking over. Not really interested in taking any form of risk.
Plus, if you know next to nothing about beer, why introduce new products that you don't know how to push- or god forbid, people start asking you about. Best to stick to the same old story. No risk that way
So while lots of consumers will be swayed by marketing, fear of the unknown, comfort in current brands & drinking patterns, a lot of people are constrained by choice or lack of. So there needs to be more done to convince publicans to stock craft beer. How, when & by who- I'm not so sure.