There are many factors. However, I would say the 'cost and capital outlay' is a primary one. Most small businesses may not have such large amounts to outlay. In addition, a business needs the right skill-sets to operate such systems thereby requiring more investment in staff development.
I agree with Paula Costa and Sachin Suknunan - Additionally, I add the robustness of IT platform, as well as & offerings of ERP installation & maintenance offerings, in small business environment.
Small businesses often hesitate to adopt ERP systems due to high costs, complexity, implementation challenges, and concerns about the time and resources required for training and integration.