I'm not sure that injuring a pregnant woman and causing the loss of the pregnancy is necessarily more injurious than what is possible with other people. But it is, justly, recognized as serious. When tallying the harm that was done, the loss of a pregnancy represents more harm. Similarly for anything else. If someone does not have a finger, that finger cannot be injured. But when tallying the harm done to an individual, inuring their finger represents more harm.
In general, when a crime is committed against a particularly vulnerable member of society, that crime gives the judicial system additional insight into the criminal's mindset, and may justifiably carry with it a harsher penalty. Kicking a fit man in the stomach does not represent the same mental state as kicking a pregnant woman in the stomach, simply because the attacker had much more clear expectation of causing wide ranging harm.