1. The first reason for this is compliance. Most large companies are worried about compliance and trust only comes with many years of successful deployments. For a new entrant to be recognized can take upto 10 years and for clients to accept the solution as a solid one.
2. The second reason is complexity. When you consider the scale and complexity of an implementation, most new ERP companies do not understand the effort goes into pre-sales and going through hundreds of pages of RFPs. There are system integrators, consultancies and entire ecosystems locked in to this duo-poly.
But things are changing. We run an open source product (https://erpnext.org) and we have seen people are looking to move away from the SAP-Oracle duopoly. Innovative companies where the CTO has as much as a say as a CIO in such decisions are willing to look outside at Open Source. It is still very early days, but this space looks exciting right now.