There are practical reasons why the JWs limit the New Covenant to the 144,000. It would undermine their multiple salvation plans during the millennium.
The New Covenant is the quintessential arrangment between the Almighty and all Christian believers, not merely a select 144,000.
33 “For this is the covenant that I shall conclude with the house of Israel after those days,” is the utterance of Jehovah. “I will put my law within them, and in their heart I shall write it. And I will become their God, and they themselves will become my people.”
34 “And they will no more teach each one his companion and each one his brother, saying, ‘KNOW Jehovah!’ for they will all of them know me, from the least one of them even to the greatest one of them,” is the utterance of Jehovah. “For I shall forgive their error, and their sin I shall remember no more.” (Jer. 31:33, 34)
It is the Great Crowd of Jehovah’s Witnesses’ stated purpose to teach the resurrected during the thousand years how to know God and Jesus based on written decrees, but this is at direct odds with the New Covenant where the Almighty is changing hearts from the inside out through the power of the Holy Spirit, not through the imposition of more externally imposed Mosaic-like laws and decrees. Through the sacrificed blood of his son, Jesus, God forgives sin and redeems man by virtue of grace through their faith in Christ and his sacrifice. He does not teach man to be sinless or perfect, nor does he forgive sins in the Christian era based on their successfully passing tests in another life. Forgiveness through Christ’s sacrifice under the everlasting New Covenant supplants the Jehovah's Witnesses' “educational” route and renders it useless with respect to their stated purpose.