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If you search Wikipedia or Erowid for the chemical structures of 2Cs and NBOMes, you'll find that while the NBOMes contain the core 2C skeleton, they have an additional aromatic ring on the amine part of the phenethylamine core. That is enough to significantly change their absorption profile, tendency to cross the blood brain barrier, and affinity for the target binding site. So while they are related, it's not "siblings" like the 2Cs are to each other. It's more like "cousins".

Small structural changes can result in large changes in effect and toxicity. Moving to a new series like NBoMe which may share some properties, from halogenated phenylethylamines can mean very different effects.

Likewise sulfur compounds like the 2C-T series may be very different.

The fact that they all start with 2C, indicating a phenylethylamine, is not enough of a discriminator for their action.

Well asked. The two commenters below give better answers than I would write. Basically, because they're quite different chemicals.