Very good question, seems to be a strange decision. Maybe it doesn't meet what they deem up to the standards to include it in RDS due to the nature of how the backend is hosted (just a guess). Could be something as simple as the dependencies the extension requires are incompatible with what the RDS servers run. It would be very nice if the RDS team had some guidelines on what it takes to get an extension accepted for RDS with a good framework people can follow to ensure their extension complies with the security requirements, etc. Then have a way to submit an extension to be included and have it be reviewed and given guidance on any changes needed to comply (for a fee potentially).