Because care is taken for their safety. Those that pay allegiance to Christ as a King shall be sure of his protection. It is the immortal soul that Christ preserves, the inward man, which may be renewed more and more when the outward man decays. They have therefore reason to be glad, being thus safe. Because provision is made for their comfort.
Light is sown for the righteous, that is, gladness for the upright in heart.
The subjects of Christ's kingdom are told to expect tribulation in the world. God's goodness shall be sure of a harvest in the appointed weeks. Those that sow in tears shall, without fail, reap in joy, Ps. Gladness is sure to the upright in heart, to those only that are sincere in religion. The joy of the hypocrite is but for a moment.