That's a reasonable hypothesis to draw, but my personal opinion is that it's incorrect.
The Sprint manual shows that playback of video is limited to 6000 kbps -- and anecdotal evidence so far shows that playback (of externally loaded content) above this bitrate will result in frames dropping. This would lead one to believe the more likely reason is that it's a processing limitation.
That said, bitrates taken in isolation are extremely misleading. I've transcoded movies to 720p, H.264, with a bitrate of 5 Mbps and they look great. The problem with the evo is not necessarily the bitrate, but the lack of the H.264 codec encoding, and some possible combination of the sensor and the lens.