gRPC had a graduation application open for 3 years. It was rejected very recently: https://github.com/cncf/toc/pull/300.
Reading between the lines, it sounds like the main problem is Google's tight control over the project. Apple contributes to the Swift implementation and MSFT drives the native .NET implementation, but there's little non-Google input in decision-making for Go, Java, C++ core, or any of the implementations that wrap core.
More subjectively, I'm impressed by the CNCF's willingness to stick to their stated graduation criteria. gRPC is widely used (even among other CNCF projects), and comes from the company that organized the CNCF - there must have been a lot of pressure to rubber-stamp the application.