Probably because it's clunky as hell and opens up more chances to get Holy locked for low return. Holy Paladins are still in a terrible spot either way. I've been Pally Healing since I installed WoW almost 10 years ago and WoD has made the spec unplayable for me. My heals (when not kicked) heal for nothing and if I do get kicked I die instantly. It's insane. I understand instant casts and mobility were 'out of hand', but I laugh about that when I see MW monks flying around healing like crazy or the crap Disc Priests do.
Blizz just needs to bite the bullet and make abilities work one way in PVE and one way in PVP. Or even give us different abilities in PVP so they can stop this circle jerk of 'fixes' and 'tweaks'. They hurt both aspects of the game (ie PVE and PVP) and push people away from playing what they want. I don't want to play Ret in PVP, but as I leveled a Paladin first and don't feel like power leveling an alt right now I am stuck PVPing as Ret to not be total trash.