Traditionals aren't as popular for the same reason that standard transmission cars aren't either. Simpler to operate. One hand opening versus two. Locking blades are like anti-lock brakes, you don't need it but once in awhile but you're really glad it's there when you do. Traditionals are also generally more chunky and heavier than a modern knife with the same general blade size. (Buck 110 7.2 oz 3.75 inch blade, Spyderco Endura 3.7 oz 3.8 inch blade) As a note there seems to be a current trend towards overbuilt needlessly heavy pocket tanks that simply boggles the mind of knife enthusiasts such as myself that only use knives to cut things. 
There will always be collectors and users of old-timey knives. As 42Pixels mentioned they are currently undergoing a resurgence in popularity. Nostalgia has always been a good way to market and sell products. Many people find comfort in being connected to their past by carrying a knife, "Just like my grandpa used." I myself am not immune to this as I recently received several knives belonging to my now deceased grandfather. They have been cleaned, sharpened, and have entered my EDC rotation. (Mostly church and the like.)  
I too have often longed for the dual blade knife for the exact reason, one general purpose blade and the other kept extremely sharp as a scalpel. Alas, no manufacturer has figured out how to make a single knife where both blades are easily accessible with one hand, both blades lock, and they can automatically be differentiated from each other without looking at them.  (Such a knife might be more like a lightsaber, a cool idea but technologically a relative impossibility.) Until then, I generally carry multiples, with the scalpel blade being on a SAK hanging on my keychain. 
Another personal reason I have for not buying more traditionals is the lack of blade steel choices available in the more readily available brands. I can buy a Spyderco in S30V, S35VN, S90V, Elmax, CTS-XHP, S110V, M4, and others all currently available and currently in regular production. How many steels does Case offer in their traditionals? They have used a couple different ones, but not many and not readily available.
Grizz