“In other words, because the Church failed to win any significant following in Israel and was about to be eliminated shortly.”
No, the Church had won a number of Jewish converts - 3000 on Pentecost.
Acts records “1Now when they had passed through Amphipolis and Apollonia, they came to Thessalonica, where there was a synagogue of the Jews. 2And Paul went in, as was his custom, and on three Sabbath days he reasoned with them from the Scriptures, 3 explaining and proving that it was necessary for the Christ to suffer and to rise from the dead, and saying, “This Jesus, whom I proclaim to you, is the Christ.” 4And some of them were persuaded and joined Paul and Silas, as did a great many of the devout Greeks and not a few of the leading women.”
More important, “15As I began to speak, the Holy Spirit fell on them just as on us at the beginning. 16And I remembered the word of the Lord, how he said, ‘John baptized with water, but you will be baptized with the Holy Spirit.’ 17If then God gave the same gift to them as he gave to us when we believed in the Lord Jesus Christ, who was I that I could stand in God’s way?” 18When they heard these things they fell silent. And they glorified God, saying, “Then to the Gentiles also God has granted repentance that leads to life.”
The question wasn’t number of converts, but who was first in line.
First biblical numerical claims are often exaggerated, contradictory or even impossible. They simply cannot be taken on their face value. We have one instance where Peter was addressing thousands of people without a microphone or a megaphone. It doesn't take a rocket scientist to realize that not all these people could have heard what he was saying.
Second, the Jerusalem church was pretty much routed after the stoning of James c. AD 64. We also know that followers of Jesus were being thrown out of synagogues (as Jesus told them they would be) in Israel, and actively persecuted by the Jews (Paul being a witness to that), as well as the Romans.
Third, even if 3,000 converts did exist, how does that number stack against those who rejected Christianity? Judaism even officially rejected Christianity and Christian books in Jamnia, so—as such—Christianity never took any roots in Israel as long as it was in Jewish hands. The Church was effectively dead. It's only chance of surviving was for Paul to sell the story to the pagan Greeks.
Acts records “1Now when they had passed through Amphipolis and Apollonia, they came to Thessalonica, where there was a synagogue of the Jews...
I am not sure what this is supposed to prove. Thessaloniki is in Greece, not Israel. And if Paul was allegedly reasoning from the scriptures in the synagogue there that “This Jesus, whom I proclaim to you, is the Christ” I really wonder what passages was he using to show them that this is who claimed Jesus was. Of the seven biblically established requirements for a Jewish messiah, Jesus fulfilled one—being Jewish!
And what is "great many?" How many Jews lived in Thessaloniki and how many could fit into the synagogue? And what were the "devout" Greeks doing in Jewish synagogues, and since when did Jewish women count form much in those days? The whole story is so blatantly naïve that it's almost comical, let alone believable.
More important, “15As I began to speak, the Holy Spirit fell on them just as on us at the beginning...And they glorified God, saying, “Then to the Gentiles also God has granted repentance that leads to life.”
How did he know the Holy Spirit fell on them? Was he hallucinating in another trance as in verse 5, or seeing an angel standing in his house, as in verse 11, combining visual with auditory hallucinations? So I am sure he also "saw" the Holy Spirit "fall on them" too.
Obviously, when the Book of Acts was written it was written not for the Jews but for the Greeks, with the express purpose to convince the Greeks that they were included in God's plan as well, because Jesus surely never, ever taught anything even remotely close, or hinted at such a ridiculous aspect of his mission on earth!
The question wasn’t number of converts, but who was first in line.
The question was who was left in line. The Church was catering to them and Acts in particular makes sure the story reflects that necessity.