the reason why is we are adopted into the family of God. and a adopted son has all rights as a nature born son correct? So if salvation came to the Jews which it did and the Spirit came on them as HE did . the same salvation saved the gentile. The Same Holy Spirit empowered the gentiles as HE did the Jews . yes Peter did explain this wonderful event in Acts 2. But he also said this was for all that the Lord should call.
Messiah. That has not happens while Pentecost! And also the Spirit was not given to all Israelites during Pentecost. Only to the believers in Jesus Christ. Jews and Gentiles.
But I don't see Joel 2 compleate fullfilled and I see also not that Christianity is instead of Israel Gods folk. And how you explaine that only one part of Christianity has the sign gifts and the other part not. But all claim to have the Holy Spirit?
This makes no sence because Joel 2 describes an act which is going out from God without influence of the people who will get the Spirit. This was not happend at Pentecost and not till today!