Why not "King of Heaven comeS" ?

It seems to be an imperative as in "Come here, please."
Here's an excerpt from the lyrics:

"King of Heaven, come down
King of Heaven, come now
Let Your glory reign
Shining like the day
King of Heaven, come
King of Heaven, rise up
Who can stand against us?
You are strong to save
In Your mighty name
King of Heaven, come"

I didn't place the commas but copied & pasted it as I found it online but their use makes sense.
There are three variations of the line in question:
1. King of Heaven, come down
2. King of Heaven, come now
3. King of Heaven, come
It seems clear to me that the third iteration is just as much a request (grammatically an imperative) as the other two.
Hence, the usage of the imperative rather than the indicative (which "comes" would be).

PS: You might wanna have chosen the Language section for this question rather than Religion...

It is asking the King of heaven to come.

Maybe it's an order. "King of Heaven, Come!"

Maybe because it is a prayer asking Him to come into our lives?

How often do you say "Comes here" to someone you want to approach you?

Because "the Kingdom of heaven" 

>>>CAME<<< on the Day of Pentecost (Acts 2;2-4).

Jesus said "If I cast out devils (((BY THE SPIRIT OF GOD))) then (((THE KINGDOM OF GOD))) is come unto you" (Matthew 12:28).

Jesus promised "In My Name ye shall cast out devils..." after Holy Spirit baptism in Mark 16:16-17.

As stated in Matthew 12:28 The Kingdom of God is the Spirit of God Who is the Holy Spirit Who is Jesus (John 14:18) 

Who is HERE NOW 

Otherwise you call JESUS A LIAR in Mark 16:16-17 when HE PROMISED we'd be able to "Cast out devils in His Name."

If the Kingdom of God is NOT HERE NOW, then "Christ doesn't dwell in you and you are none of His" (Romans 8:9) 

"Wherefore henceforth know we no man after the flesh: yea, though we have known Christ after the flesh, yet now henceforth know we Him no more"  (II Corinthians 5:16)!

Christ is Spirit!  

The Holy Spirit (John 14:18)!  

The "Spirit of God" (Matthew 12:28)!

Who is the "Kingdom of God" (Matthew 12:28)!

Who CAME back in the glory of the Holy Ghost (Acts 1:2) on the Day of Pentecost (Acts 2:2-4)!

For "If the Spirit of Christ doesn't dwell in you then you are none of His" (Romans 8:9)!

Matthew 6:10 has been FULFILLED!