Because 84.1 in the NHL Rule Book clearly states that the only times a player "substitution" is allowed after an icing it to replace a goalkeeper who was pulled in order to gain the extra attacker, to replace an injured player or if a penalty has been assessed which will alter the on-ice strength of either team.
So a player who is not wearing the proper padding to stop a puck without risking serious injury to his face, arms, legs, feet, mid-section or any other area not sufficiently protected (which can be several areas considering the differences in padding for skaters versus goalies) should be
required to get into the net after an icing that took place with a team's goalie pulled?
Yeah have fun getting that one past the NHLPA...
The reason goalies are protected from body checks is the same reason a player should not be "required" to play goalie without the proper pads. The equipment of a goalie is designed to stop pucks while transferring the force of the impact across a large piece of padding. The equipment of a player is designed to protect the body from hard hits, and the relatively rare (compared to a goalie) times when they are hit with a puck.
I suppose they could allow the sixth skater to put on goalkeeper gear, but that would take so much time it would be ridiculous. The rule is fine the way it is. The point of the lack of substitution of skaters after an icing is to further deter a team from simply throwing the puck down the ice. Not only is there a faceoff in your own defensive zone, but you're going to be tired, too. What does the goalie have to do with that? A team should now have to risk letting up an easy empty net goal because they iced the puck with their goalie pulled?
That's just ridiculous.