Short Answer. Because the risen and glorified Lord revealed the gospel of the grace of God to Paul alone. Not Peter or the other apostles.

The Lord revealed to the apostle Paul something new a “mystery” or secret from the Greek word mysterion (μυστήριον). A secret that had been hidden in the mind of God since the world began. This is why the Holy Spirit inspires the apostle Paul to use the personal pronoun “My” when referring to the gospel of grace. This new message was unique to Paul.

Adam, Noah, Abraham, Moses, Isaiah, David, Jerimiah, not even Peter, and the apostles knew anything of the gospel of grace. If they did then Paul’s epistles would be a lie. Making God a liar and can not lie.

Background

From the time God called Abraham to make a people for Himself up to Paul’s conversion, there was no message of salvation to the Gentile world. The message from Genesis 12 all through the old testament, the four gospels until we come to Paul in Acts 9 is directed to Jew only. With only 9 gentile exceptions over a 2000 year period. And the message prior to Paul’s “My Gospel” was that a Messiah, a King would come to Israel, crush their enemies, and set up a glorious earthly kingdom. Israel would be the preeminent nation above all nations with Christ ruling from David’s throne in Jerusalem (Isaiah 2:2-5, 9:6-7, 11:1-16; Jeremiah 23:3-8, 30:8-24; Ezekiel 36:21-38, 37:1-28; Zechariah 14:8-11). Gentiles were always on God’s mind for salvation but Gentile salvation was to come through Israel (Zechariah 8:20-23; Isaiah 60:1-3).

Every Jew knew this and was waiting for their Messiah, their King, and the kingdom. Not the Gentile world, they were all pagan, knew nothing of these promises, and hated the nation of Israel. But we know from scripture what happened, all in God’s preordained design (Acts 2:38). Israel’s Messiah did come to the nation preaching the kingdom of heaven is at hand (Matthew 4:17). He performed many miracles to prove to the nation that He indeed was the Messiah. But they rejected Jesus as their King and said we will not have this man to rule over us (Luke 19:14). And together with the Romans (Gentiles) crucified Israel’s Messiah (Psalm 2:1-3).

With the climax of Israel’s rebellion in Acts 7,8 and the stoning of Stephen. God calls out one man, Paul to be His chosen vessel to bear His name to the Gentile world.

‘But the Lord said to him, “Go, for he is a chosen vessel of Mine to bear My name before Gentiles, kings, and the children of Israel. ‘

Acts 9:15

God is doing something completely different. For the previous 2000 years, there was NO message of salvation or doctrine for Gentiles. But now, God reveals to Paul alone a “Mystery”. That now, salvation was to both Gentiles and Jews apart from the Mosaic law and temple worship. By simply believing by faith alone in the gospel of grace alone.

‘Moreover, brethren, I declare to you the gospel which I preached to you, which also you received and in which you stand, by which also you are saved, if you hold fast that word which I preached to you—unless you believed in vain. For I delivered to you first of all that which I also received: that Christ died for our sins according to the Scriptures, and that He was buried, and that He rose again the third day according to the Scriptures, ‘

I Corinthians 15:1-4

This was unheard of for 2000 years! Israel’s God going to the Gentile world with a message of salvation apart from Judaism. Yes, the God of glory opens the windows of heaven and pours out His grace on all mankind. And God chose one man alone, Paul, to reveal this new message, the gospel of the grace of God to the Gentile world.

‘But the Lord said unto him, Go thy way: for he is a chosen vessel unto me, to bear my name before the Gentiles, and kings, and the children of Israel: for I will shew him how great things he must suffer for my name’s sake. ‘

Acts 9:15-16

Paul’s “My Gospel”

Paul uses “my gospel” 3 times in scripture. And remember this isn’t Paul’s idea the Holy Spirit inspires Paul to write.

When the Lord Jesus Christ sits in judgment at the Great White Throne. It will be the gospel given to Paul by the risen Lord by which the lost will be judged.

‘in the day when God will judge the secrets of men by Jesus Christ, according to my gospel.’

Romans 2:16

In Romans 16:25 Paul again uses the personal pronoun “My” with the gospel. This passage also delineates that the gospel given to Paul was kept secret since the world began.

‘Now to Him who is able to establish you according to my gospel and the preaching of Jesus Christ, according to the revelation of the mystery kept secret since the world began ‘

Romans 16:25

The apostle Paul uses “My Gospel” for the third time in 2 Timothy 2:8. Where he speaks of the resurrection as the culmination of God’s eternal purpose. Which was that the resurrection of Jesus Christ would be the means of salvation for ALL mankind not just Israel.

‘Remember that Jesus Christ, of the seed of David, was raised from the dead according to my gospel, ‘

II Timothy 2:8