The amount of people able to afford 4k and 4x titans vs the amount of people able to afford 4k and 2x290X's. Not everyone will have as much disposable income as you to spend on these things Kaap. You need to see it from the other side as well.
Some people upgrade from 720/1080P to 1600P or higher and make do with lower settings. Just because someone upgrades their screen does not suddenly mean that they have to run the absolute highest settings or its pointless. I bet not everyone who upgrades to 4k will do so on 4x titans. Some people who finally do make the jump to 4k might be looking for other cheaper gpu solutions that still allow them to run with max game settings but maybe with a bit of AA sacrificed, which is not needed at 4k anway. Those people will be interested in a 290X at £430 or so.