The usual cause is requirements written as a feature list instead of outcomes. Vendors respond to what you ask for, and a long checklist rewards the longest product, not the best fit.
Start from outcomes
Define the outcomes the business needs and how you will measure them. Then write evaluation criteria that reward evidence of those outcomes: references, demonstrations against your scenarios, and delivery plans tied to your constraints.
Changes after an RFP is released cost time and invite challenge. Test requirements before release, not after.
Review the requirements with the people who will use the system, and with legal and finance, before issue. In the public sector, that discipline is also your best defence if an award is challenged.