The MPAA requires much tighter security on higher than 1080 television/streaming. To put on a linear channel the stream would have to be in MPEG4 or higher. On-demand or a streaming app service that is IP streamed DoCSIS level must be at 3.1 or higher (the end to end encryption removes possibility of man-in-the-middle intercept of non encrypted stream). I can understand that being a requirement. I have no inside data of Comcast's plans but my guess is any new set top box will be receiving an IP stream until the particular area converts their local plant to 3.1 DoCSIS.
Edit: had to rework that whole paragraph.