A: Because it has been the preferred weapon of modernity, over the past half century.
In opposing Lust, Christians have been dueling the enemy, using a weapon chosen by the enemy.
Lust is not a major sin in Christianity, not like Pride; but Lust is a major virtue for modernity - and a stalking horse for the other sins.
The gratification of Lust is used to justify more freedom of lifestyle, and this is used to reinforce Pride - which achieves the rest.
Leftism began by promising the mass of people food and shelter.
When food and shelter had been provided to everybody, albeit by capitalism not Leftism, Leftism shifted to promising the mass of people gratification of their sexual desires.
(Of course the primacy of Lust goes back a couple of hundred years among the classes who already had enough food and shelter.)
Leftism has emptied human life of all meaning and purpose and left humans isolated and atomized - yet all propaganda is directed against the one and only true solution to this self-created problem.
So distraction has become the primary mode of human existence, and this is where Lust comes in.
The distracting power of Lust, and its utter un-realism, has been built-up by modernity to a level where the reality of its psychological operations is so vast and encompassing that it is unbelievable when stated clearly.
Or not so much unbelievable, as humiliating.
(Are we really such pathetic creatures as to be led by the nose with ludicrously fabricated fantasies of Lust? Yes we are. Look around.)
Lust powers the modern world.
In our world as it has become, without Lust nobody would get up in the morning, nobody would talk to others, nobody would work, nobody would buy stuff, nobody would have anything to look-forward-to - people would have nothing to do, no reason for doing anything and - much worse - nothing to think about.
Of course Lust is very seldom actually gratified; yet the merest remote possibility of gratification is the most effective of all distractions.
Christianity has responded to this tsunami of Lust by trying to hold the line on specific points-of-Law - but with the exponential growth of mass media, mainstream Christianity has been like a pebble rolled by the tide.
Only those devout groups who cut themselves off from the mass media, who are actively hostile to modernity, have resisted.
So, the primacy of Lust is a consequence of the decadence that ensued from prosperity; since when it has become foundational to modernity as the primary motivator, Lust is that which lies behind most of the carrots offered by contemporary culture.
And mainstream Christians have been utterly unable to resist Lust because they are addicted to modernity.
Only those religious groups who have explicitly rejected modernity have resisted the primary distraction of Lust; and these are precisely the only organized groups which are now thriving, growing.
The West has been crazed by Lust, because Lust is all that it has.