Well, by AndyB words:
He cites two functions the Devs see tanks doing.
Following your tank around because he offers you protection as a “push-leading protector” is just the easiest strategy to VISUALIZE and follow up (playing gathered with your tank), specially with Rein that just incorporates this function the most.
Now, following up a “disruptive line breaker” tank is a harder strategy, not easy to understand or visualize how this is gonna help you individually because this doesn’t protect you directly. Wrecking Ball incorporates this function the most and people can’t just follow him around, instead people gotta play more carefully positioned while they take advantage of Hammond’s disruption (CC’ing enemies into off-angle exposure) to inflict damage and secure kills.
That’s why in metal ranks there’s a Rein preference/demand by non-tank players to work with, it’s easier to follow up something they can directly visualize the benefits.