Because of the context of the chapter and who it is He is speaking to. This context determines the time frame of the coming He speaks of here...
Matthew 10:5, 16, 23
These twelve Jesus sent forth, and commanded them, saying, ...But go rather to the lost sheep of the house of Israel... Behold, I send you forth as sheep in the midst of wolves... But when they persecute you in this city, flee ye into another: for verily I say unto you, Ye shall not have gone over the cities of Israel, till the Son of man be come.
Then you have the fact that Jesus assured them that there were some standing in their midst who would not die until they saw Him come in His kingdom (as He says, "with His angels in the glory of His Father.") Matt 16:27, 28
I hope the Lord comes a final time to put an end to the wickedness that permeates this world, I honestly do (and God knows this) but I cannot find it taught in the Scriptures as such.
Blessings.