Because, From Deep Within Him, He Follows Hard After God, God's Right Hand Upholds Him, So That Those Who Are Seeking To Destroy Him Will Themselves Be Destroyed (Psalms 63:8-10).
From David's inner thoughts springs inner action. His inner life follows hard after God. This is why God's right hand upholds him, and deals firmly with his enemies.
‘My inner life follows hard after you,
Your right hand upholds me.'
And those who seek my inner life, to destroy it,
Will go into the lower parts of the earth.
They will be given over to the power of the sword,
They will be a portion for jackals.'
‘My inner life follows hard after you.' This is literally, ‘my inner life cleaves after you'. He clings on to God at all costs and follows Him, being bound to God by God Himself. To him God is everything. And because this is so God's right hand upholds him. The right hand is indicative of the most powerful hand. He is upheld and sustained by God in His Almightiness. And thus he has total assurance that his enemies cannot prevail.
Those who seek his person in order to destroy him will instead find that they themselves will go to the grave. While he, in his inner heart, goes upwards to God, they will go downwards into the grave world, swallowed up like Korah and his fellow-rebels who sought to usurp God's anointed priests (Numbers 16:31-33). Whilst his life is given over to God, their lives will given over to the sword. Whilst he is God's portion, they will be portions for scavengers, who will eat their dead bodies. Their future is bleak indeed. Such is the portion of those who ill-treat God's people.
‘They will be given over to the power of the sword.' This is literally ‘they will pour him out to the power of the sword', with the ‘they' being God's mysterious instruments of justice.
‘They will be a portion for jackals.' The fate of those who rebel against God is often depicted in such terms. Compare Ezekiel 39:4; Ezekiel 39:17-20; Revelation 19:17-18.