There's a few basic reasons.
1. Preparations have no drain when used. Resisting drain has ALWAYS been one of the major limiting factors of magic in SR. Having to balance the needs of the moment against the very real possibility of knocking yourself out of the fight, and maybe the entire run, is a very real part of the game balance. Alchemy as it stands skirts this balance, but the limitations are such that it is still impossible to spam magical attacks. This helps, but doesn't completely solve the issues regarding magic in the field, especially with certain things.
2. Command preparations require the same action on the part of the caster as a spell, meaning you can set off a command preparation, or cast a spell, or attack, but not two or more of those things at once. Now, you want healing at range? Get a couple command preparations, plant them on your team, and set them off when you need to. Since you're still using your action to do that, the balance issues are minimal. But the point, Hobbes, is that it is the MAGE using their action to set off the preparation, not the sammy, meaning the mage can't throw a fireball, summon a spirit, or do any of the other innumerable things they can do to mess your life up until the next pass.
3. The risk of getting tagged while getting to injured team mates is part of the game balance, you know. That is second major limiting factor to healing in SR. This limit on healing helps shape the combat the way it is. Especially when considering scaling wound penalties. It is one of the fundamental conceits of the game system that separates it from D&D and other similar games, where a fighter can charge into a horde of enemies and rely on the healer standing safely 30' back to keep him from dying.
4. Restricting healing spells to command trigger prevents groups from just buying a dozen healing potions each and slugging them back one after another during a gunfight. Yes, the healing doesn't stack, but that's only taking into consideration the SAME set of wounds. You get shot, heal, and get shot again, that damage from the second shot can still be healed with a spell. Without the mage having to use an action to activate healing preparations, that can become very bad, very fast. And any of you who tries to tell me there aren't groups that wouldn't do this if they could to bull rush through encounters is either hopelessly naive or a dirty, dirty liar.
5. Geek the mage first. Another of the key limitations on magic in SR is that if you kill the mage, the magic stops. You kill the mage, not only can he not heal himself, but others can't use his healing preparations to bring him back into the fight. And that is not a small thing. Remember, if your team can heal up their mage like that, so can the Azzie corpsec with their blood mage.
6. For all the reasons that hackers can't stay in the van any more. The real reasons, not things like Noise.

tl;dr - While it sounds small, this kind of change affects a lot of things at a basic level. Being able to heal after each set of wounds without crippling drain penalties alone should be enough to make the point.