Well, HD2 only sold a little over 1 million copies, Mafia sold over 2 million copies so you can see right there why they would first invest in the Mafia title.
Also, I wonder if maybe marketeers think the WW2 hype has been overused in the past two decades. I mean there have been so many games taking place in WW2, most modern shooters have left the WW2 scenario for more modern and futuristic (multiplayer) gameplay.
But of course you don't have to ask the question if we would all like to see HD3 happen...it is asking the obvious. Maybe one day it will come but personally I don't think so.
There was a petition before to make HD3 but as you can see below WAY TO FEW people ultimately signed it, not a convincing signal that there would be a good return on the investment.
HD3 petitionhttps://www.change.org/p/2k-czech-make-hidden-and-dangerous-3