Based on my personal experience, it's a matter of education. They probably need to be onboarded to ecommerce and also be provided with solutions that help them establish that channel successfully. Fulfillment being on of the biggest nightmares for novice ecommerce owners.

I don't have much background information on your geographic area, but I'm assuming that your local folks have a defined market that they probably don't want to expand, but just to cater to the local community.

Now, on the other hand, what type of brick and mortar are those? Some of them might be eligible for an easy transition to the online world, but some others won't, and not because it's impossible, but because they probably won't meet demand based on their production scale. Or perhaps, they have products that are not easy to deliver and maintain fresh (like foods.)

You may show the benefits by presenting them with case studies on how similar businesses have made it big with ecommerce, and what steps they took to accomplish such project. Lay out sample business plan, operations model, revenue model, distribution, etc.