Because it's meta. Any "tank" champion is just going to be changed to have % health-based damage now because that's the only way Riot can think of allowing tanks be as tanky as they are and still do damage throughout the game. If this continues, you want to see how any melee champion's skill set will look in the future? Throw a % health somewhere in there.
I'd rather get the 15% armor shred for your team's physical damage than % health damage for you only. The no-collision skillshot change would have been great, but changing the on-hit to % health for Shyvana only is ****ing stupid.