The number of Pokemon with healing moves available to them are limited. Plus, you can just switch out of Heal Block. All of the Pokemon who get it cannot spare the moveslot. Reuniculus, Gothitelle, Beheeyem, these Pokemon would much rather have coverage or stat boosters. Or their own form of recovery. Solrock, Lunatone, Bronzong, Claydol, these guys need the spots for their Rocks and their Trick Rooms, then fill it up with attacking options in order to not be Taunt bait. Celebi has a huge number of more useful moves available to it. Shedinja can't waste time using a move that won't let it hit well with priority. Heal Block will likely never come in handy. The other moves these Pokemon can learn and should use are required for their success.