Why is the EOS M6 Video Bitrate lower than the EOS 1200D/T5?
That would be a great question for Canon, because only the manufacturer can provide the reasons considered when producing a new model. I only can venture a guess that the M6 is a smaller camera that presumably is a lower-level compared to full-body T5. Maybe higher bitrate results in more heat, and the smaller body may have issues with dissipating it. Your guess is as good as anyone else's. It seems that the M6 uses the same H.264 as the prior models instead of the more efficient H.265, so seems that this is not the reason for decreasing the bitrate.