Because the former is not the intended path to victory for the martial while the latter very much is the intended path to victory for the caster?
Anytime the monster's AC and hit points doesn't matter for a martial, something's gone wrong. It is far from the first time a dev team finds out that their "alternative" combat system (whether it's a separate system for unarmed combat, weapon duels or grappling etc) means minmaxer dump the regular combat system. (Likewise if there's a "spell duel" system that bypasses regular spell resolution. Either it sucks or it is OP)
In short, it's practically impossible to achieve total parity between competing systems. (Unless, you know, you make the systems work identically, which kind of defeats the point). Put simply, either grappling is better than regular combat or it is worse. Either everybody uses it or nobody uses it.
Again and again AND AGAIN we have a situation where it comes as news to a dev team, who hastily has to remove or nerf the alternative system so it's no longer used by minmaxers. This is just another such day. It has happened countless times before and it will happen countless times again.
If it sounds like I've seen it before it is because I have. This question ("why did they take away my toys?") was asked ten years ago. It was asked twenty years ago. It was asked thirty years ago.
The answer remains the same.
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