Christ had promised His apostles that He would come again in their generation for those who were EAGERLY WAITING for Him to come. It is not a promise to come again and again. It is a promise to come again one time to those first century saints (the Bride of Christ), who were EAGERLY WAITING for Him. Once He fulfilled that promise in AD 66, it was done.
The first century saints, who were suffering under much persecution for their faith in Christ, increasingly in the 1st century, and especially during the Great Tribulation of AD 64-66, were EAGERLY WAITING for Him to come and rescue/save them.
“23Not only that, but we also who have the firstfruits of the Spirit, even we ourselves groan within ourselves, EAGERLY WAITING for the adoption, the redemption of our body.” (Rom. 8:23),
“7so that you come short in no gift, EAGERLY WAITING for the revelation of our Lord Jesus Christ.” (1 Cor. 1:7).
“28so Christ was offered once to bear the sins of many. To those who EAGERLY WAIT for Him He will appear a SECOND TIME, apart from sin, for SALVATION.” (Heb. 9:28).
“37For yet A LITTLE WHILE, and He who is coming WILL COME and WILL NOT TARRY.”（Heb. 10:37).
[For a more in-depth study see eschatology “Study Series 14 Lesson 4 Resurrection: Expectations/Experiences”]