Porrohman got it spot on - with EDI, GLA and ABZ well established, where is the business case for substantially re-developing Dundee? How do you get your money back?
If Dundee can't attract pax who are prepared to pay a high enough premium for the convenience/time saving of not having to travel to another airport, it is going to need a public subsidy to retain airline services.
Also, how many of those people who don't know about Dundee airport regularly fly to London? There's a difference between the awareness of the general public and that of your target market.