In a word: simulator-sickness
We are very concerned for the well-being of clients. We also want our product to be effective for the majority of people, including those susceptible to simulator-sickness. Additionally, we believe it isn’t possible to effectively train public speaking while experiencing simulator sickness effects.
A lack of full head tracking has been long identified in research as an important factor in the occurrence simulator sickness. Phone based HMDs do not have full head tracking. They detect only rotational head movements and may not be accurate enough in those measurements. While for short exposures this may not be a problem, public speaking is generally longer than a couple of minutes. Over time, this means many will start experiencing simulator sickness. Many effects of simulator sickness are similar to effects of the stress of public speaking, meaning people may not recognize the seriousness of those indicators quickly enough, leading to a harsher experience.
We support systems with full head tracking, which right now includes only the PC based HMDs. There are new tracking methods in development that will allow full head-tracking of phone based systems. We will consider supporting them when they reach market.