The issue with competing against SAP/Oracle is the kind of traction they have in the market already. Their contracts are long, and companies spend so much implementing the software that they're not excited to switch.

Also, as mentioned somewhere else in this thread, ERP software (Payroll, HR, Financials, etc) are all about the details. Payroll and Financials are very complicated, which results in companies only trusting the big players (SAP/Oracle)

Disclaimer: I work for Workday