They are power centric because it’s a bit more objective and user friendly for the TR application. You have to keep in mind many have no clue how to train with power (or HR), basic training concepts, etc…It doesn’t mean they think HR is not relevant. HR is still extremely useful there are endless articles on the subject.
One of the main reasons power is more desirable as a metric over HR is for interval training. The shorter the interval the more useless HR data is. Not because it’s bad information but, because of lag. Power is more or less instantaneous while HR takes a while to elevate. So anything VO2max or higher is just more controlled with power.
However, the whole point to say VO2max is to train at or above 90%MHR. I ramp test well and trying to hold power causes way too high of HR and I basically go into the red early and shut down. HR allows me to tweak the power so I stay around 90% MHR much longer. So time in physiological zone is longer (not sure how to term it).
HR used with power can be super useful for longer interval or race TT pacing. For the same reasons as above you can easily go out too hard and power will slowly drop over time. Conversely if you just pace with HR your power will drop over time. Together you can pace closer to the upper end of your limits without going into the red or without leaving power behind.
HR is still the gold standard to determine if rested. Rested between intervals and between days, training blocks.
edit: I will say with al the years looking at both power and HR if I were a coach coaching someone who has not used either I’d want them to use power. It’s not perfect (mainly because of how we collect and interpret data) but, it’s a heck of lot easier to interpret than HR. In my opinion.