Because of the property renaming problem. If unfortunately a property is renamed and someone used the label in a parser function call, then this might break templates. This might happen if a property is split, and we don't keep the alias on both resulting properties of the splitting because of the uniqueness of aliases constraint.

Of course if the property is split there might need more drastic changes in the clients and templates, but relying on labels for stability when we have stable Pids to rely on seems like a half-baked solution to me. Especially when renaming is so easy in the UI.