So that they can be used for specific things. Specialization is needed. Maybe the new Cyclone's can do things that the Cormorant's can't.
Why develop the Apache, when the Blackhawk can be used as an assult helicopter? Well becuase the Apache is specificially an attack helicopter. While the Blackhawk can be used in an attack sence (with the right attachments), it is slow and clunky, and is better suited to troop transport.
That being said, if they are to be used for the same purpose, then yes, purchasing a completely different aircraft is stupid.