Actually DTS-MA mono assigns the audio to the center channel. Yeah, you could work around player bugs by doing dual-channel mono--and in fact, many discs do use dual-channel mono because they re-use old mono tracks created for stereo systems with no center channel. But mono coming from the center channel is actually better than coming from the front sides (assuming you have a surround setup and you're choosing not to upmix--how much better depends on placement), and it uses less space-on-disc, which for a lossless track is nice. It's not a hill to die for, but I don't want the blame misplaced.

It's entirely an implementation problem. DTS-MA mono is no more difficult to handle than any other number of channels--some designers just forgot or never knew true mono was an option.