Perhaps it’s just a matter of aesthetics, or maybe it has to do with the implications the designers want to make about the martial-minded NPC. But our best guess is that they wanted this protection fighting style to serve more as a defender, not someone who can stand in the middle of battle and take down opponents.
It also sees them as being better suited for defense than offense because their fighting style tends towards staggering enemies and blocking blows rather than launching themselves into combat like charging fighters. This makes sense when you consider that this class was created for NPCs—individuals designed as guardians and protectors, not attackers. That helps DMs easily know how to integrate these characters into an adventure without worrying whether they’re trying too hard or not hard enough to contribute in combat.
The short version is that the designer probably felt that a protector didn’t need the means to attack people—just be able to protect those under his care. In addition, it also makes sense for this class to have less capabilities when they’re on their own rather than when they have an army behind them or the support of a superior to command them.
However, this does make it difficult for those who want to play something like a bodyguard or someone strong enough to fight off the villains but also intelligent and wise enough not to leap headlong into battle without thinking about what they’re doing. The class is ultimately designed with supporting your allies in mind, so you won’t be good at dealing with mobs.
Although not a bad choice if your DM is running a campaign focused on traveling from place to place and going on big quests, the style of fighting makes it difficult for most players who want to have their character face off against villains alone in an epic battle. This means that in order to best use this fighting style, you need to have an ally behind your character—someone for the monsters to hit instead of yourself.