The decisions are taken by the stakeholders.
If Cisco does not want to support some new routing protocol then it won't be deployed. There are many similar gatekeepers.
But Cisco, Google, etc. etc. are managed by people who understand the value of being open to a wide range of ideas and companies who have not been open in that fashion have not survived as gatekeepers for very long. The IETF was formed at a time that IBM was losing that position. Cisco is ultimately responsive to its customers which are ISPs who are in turn responsive to their customers, the Internet users. So the IETF is in part a feedback loop which helps the thoughtful large companies align their interests with those of their customer's customers.
Another reason they are open to being persuaded is that changing the Internet is a collective action problem and most of the decision makers only hold veto power. To change the Internet multiple decision makers have to agree on a common course of action.
So the IETF is one of the forums where people have an opportunity to influence the ultimate decision makers. If you attend in person you have more opportunity to be influential.