Because, depending on the character, it's easy to be ridiculously strong in comparison to the Power of a firearm, especially given the higher ballistic than impact armor ratings much of the game has? Toss in Stun Gloves and it just gets ridiculous. Your average Troll punches for the same damage as a heavy pistol (firing gel rounds), and if you're not using Cannon Companion (so no access to the Close Combat manuever) will do so with a Reach modifier in his favor. Why not, even from a raw numbers/efficiency point of view, do a little unarmed combat every once in a while?

And melee is quieter, doesn't cost any money-per-shot, as well? Knocking guards out won't get you hunted as harshly (and might even fit with a character's moral code, or what passes for one)?

And if your players are wanting a martial arts extravaganza sort of game, with plenty of kung fu (or WWF, either way) action -- why not just give it to 'em? If they're eager to use descriptive unarmed combat attacks, think creatively when it comes to making their own cover (out of opponents), and stuff like that... why try to punish them for it, and herd them towards a different playstyle?

If you still have your 1st edition there are rules for grappling and sneak attacks on p71. This should cover the melee situations you described in previous posts while not getting too complicated.