DOCSIS would probably not be be specified in RFCs, but see e.g. here: https://account.cablelabs.com/server/alfresco/9b8a9bd8-dea3-...

I suppose the main reason was compatibility with existing cable headend infrastructure, but I'm also curious to know if there were ever actual TV and DOCSIS data sharing a single physical transponder. Would be a nice way of leveraging spare bandwidth due to variable bit rate video encoding!