I've asked myself the same question and here's what I found out after asking many of them as well as working with some

1) Lack of technical knowledge & skilled employees
2) No inventory system or unable to connect system to online website with ease
3) No money to implement

The best way to start is to launch them on Shopify with a small subset of their inventory only to get them started. Let them get a tasted of the sales and then it will motivate them to invest more.