Why was Mtt 17: 21 as appearing in KJV skipped in other versions of NT? [duplicate]

We read in Mtt 17:18-21 (KJV)

And Jesus rebuked the devil; and he departed out of him: and the child was cured from that very hour.19 Then came the disciples to Jesus apart, and said, Why could not we cast him out?20 And Jesus said unto them, Because of your unbelief: for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you.21 Howbeit this kind goeth not out but by prayer and fasting.

But, many versions including NRSVCE skip Verse 21 and move direct from 20 to 22:

20 He said to them, “Because of your little faith. For truly I tell you, if you have faith the size of a mustard seed, you will say to this mountain, ‘Move from here to there,’ and it will move; and nothing will be impossible for you.”. 22 As they were gathering in Galilee, Jesus said to them, “The Son of Man is going to be betrayed into human hands

My question therefore is: Why was Mtt 17: 21 as appearing in KJV skipped in other versions of NT ? Inputs from any denomination are welcome.