Because they want to disarm us.
The enemy cannot provide us with joy because joy emanates from the Divine and from what is divine within us.
Yes, the enemy can offer all sorts of amusements, distractions, pleasures, comforts, but without the infusion of the Divine, these things can never be truly joy-full - merely amusing, distracting, pleasurable, or comfortable.
The enemy's "joy" is nothing more than temptations to empty promises, but in this time and place, even empty promises strike the enemy as too "full." Consequently, the enemy has replaced its amusements with depression, its distractions with intrusions, its pleasures with pain, and its comforts with grief - all in the name of destroying joy.
For the enemy, the presence of joy is a testament to the presence of God and the presence of God's love. As such, the enemy see joy as a most heinous weapon.
Joy is not a weapon, but if the enemy believe it to be one, then be fully-armed at all times and attack them with this peculiar weapon every chance you get.
Let joy pierce depression, repel intrusion, smother pain, and assail grief.
Let the presence of God shine.
Let the presence of God's love burn.