1. We don't know
2. Because God chose the Great Commission
We see both come together here:
Acts 18: NASB
9And the Lord said to Paul in the night by a vision, “Do not be afraid any longer, but go on speaking and do not be silent; 10for I am with you, and no man will attack you in order to harm you, for I have many people in this city.” 11And he settled there a year and six months, teaching the word of God among them.
God had "many" people in this city of Corinth. A major "crossroads" in that part of the Roman Empire:
The City Of Corinth
Corinth is located in southern Greece about 50 miles from Athens, and about two miles south of the narrow isthmus that forms a land bridge between the main landmass of Greece and the Peloponnesus. The isthmus is less than four miles wide. Corinth controlled the two major harbors and thus command of the trade routes between Asia and Rome. In ancient days small ships were dragged across the isthmus on a paved road; larger ships unloaded their cargo, which was then carried across the isthmus and then reloaded onto other ships.
"One of the most important factors drawing Paul to Corinth was its strategic location on the narrow isthmus connecting central Greece to the southern land mass of the Peloponnese. Even more important than the land travel north and south, however, was the sea travel cast and west through the Saronic Gulf and the Gulf of Corinth, lapping the eastern and western shores of the isthmus respectively. The Roman poet Horace wrote in praise of 'twin-sea'd Corinth.' Ship captains plying the northern Mediterranean sea route between Europe and the Aegean Sea much preferred to bring their vessels through the sheltered waters of these two gulfs rather than to add over 200 miles to their sea journey and to risk the more exposed seas off the southern coast of the Peloponnese. Corinth stood, therefore, at a heavily traveled crossroads of the Roman empire. The idea for a canal had emerged as early as the 7th century B.C." (The Biblical World In Pictures).
Christianity in the Greco-Roman World
God's Divine Providence on display.