21 But since they have no root, they last only a short time. When trouble or persecution comes because of the word, they quickly fall away.

Read Matthew 13:21 Using Other Translations

Yet hath he not root in himself, but dureth for a while: for when tribulation or persecution ariseth because of the word, by and by he is offended.
yet he has no root in himself, but endures for a while, and when tribulation or persecution arises on account of the word, immediately he falls away.
But since they don’t have deep roots, they don’t last long. They fall away as soon as they have problems or are persecuted for believing God’s word.