Because he's writing those other epistles to Gentiles.
Galatians 2:7-9 says that Paul agreed to go to the uncircumcision, while the Twelve agreed to go to the circumcision. (AKA, Gentiles, Jews, respectively).
We've been over this. Paul was preaching to the Jews throughout his ministry.
Do you think that we preach that the gospel of grace is exclusive to the Gentiles?
That Paul went to the Jews in the Gentile cities he traveled to does not mean that Paul was sent to the Jews.
Also, I would like to point out that aside from a few short mission trips, the Twelve (what remained of them, at least) hardly ever left Jerusalem even after being given a direct order by Christ (which Jesus gave them the authority to choose to disregard if they so chose) to go out into the world.
That agreement is not the abject dividing line you are trying to make it.
Acts 28:17 (KJV) And it came to pass, that after three days Paul called the chief of the Jews together: and when they were come together, he said unto them, Men [and] brethren, though I have committed nothing against the people, or customs of our fathers, yet was I delivered prisoner from Jerusalem into the hands of the Romans.
Acts 28:23 (KJV) And when they had appointed him a day, there came many to him into [his] lodging; to whom he expounded and testified the kingdom of God, persuading them concerning Jesus, both out of the law of Moses, and [out of] the prophets, from morning till evening.
Reminder, this occurred when Paul was in Rome, a Gentile city.
Paul was not violating his agreement he made with the Twelve in Galatians 2 by preaching to the Jews during his going to the Gentiles. The Gospel of Grace is for all, both Jew and Gentile alike, for in the Body of Christ "there is neither Jew nor Greek."
But you would have to show conclusive evidence that Paul wrote to the Jews, to be able to provide evidence that Hebrews was written by Him, whereas we have multiple instances of circumstantial evidence that indicates that Paul did not write Hebrews.
Galatians 2:7-9, Acts 15:23-29, Paul's greetings in the letters we know he DID write versus the lack of such greeting in Hebrews, the usage of some terms and phrases that should be present that refer to the same thing or to the same Jesus Christ, the different styles of writing, all of these things and more point to Hebrews having a different author than Paul.