Paul left Thessalonica mostly to maintain safety for those who spoke for him before the local authorities. It is possible that Timothy and others in Paul’s company stayed behind to help establish the new church there, but Paul and Silas left for Berea. Later, the rest of the team would have joined Paul there. The Jews in Berea received the word of God and searched the Scriptures to make sure Paul was correct. However, when the Jews in Thessalonica heard about Paul’s work there, they sent representatives to Berea and stirred up the city, so Paul had to make an escape by sea.
The thing that wonders me is, why were Timothy, Silas and others of Paul’s company able to stay behind? Why weren’t they in danger? It may be that Paul kept adjusting his manner of preaching to accommodate the possibility of the most fruitful harvest from each city in light of spiritual opposition and how it works against the Gospel. During his first missionary journey Paul confronted the sorcerer on the island of Cyprus (Acts 13:8-11) and the result was Mark had left the team for Jerusalem (Acts 13:13) and many brethren there misunderstood Paul’s work in the Gospel and sent out teams to undo the work Paul had done in Antioch, Syria and Cilicia (Acts 15: 23-24). Spiritual warfare needs to be planned. Otherwise, one has to spend a lot of time cleaning up in terms of unnecessary misunderstanding. If left alone, the backlash of spiritual warfare could separate brethren.
In Philippi Paul separated his small group before attacking the territorial spirit that possessed the young woman (Acts 16:16-17), but both Silas and he were attacked by the offended townsfolk (Acts 16:19-20), the puppets of spiritual warfare taking place behind the scenes. Paul may have felt the beating and imprisonment of both he and Silas was inflicted upon one more than necessary. Apparently, Paul’s stay in Thessalonica may have been a little different, because it seems the offended Jews sought only Paul (Acts 17:13-14). Paul may have kept Silas and Timothy in the background supporting those who had already begun trusting in Christ. This understanding is further substantiated in that those from Thessalonica who had come to Berea still sought only Paul, so Paul was able to escape there by sea, while both Silas and Timothy were left behind to support the young church and establish it by laying hands upon its leadership, before rejoining Paul in Athens.
Of course these are merely my own thoughts, and some other explanation could be the truth, but this does fit the context. My only question is: so why isn’t there a letter to the Bereans? I would have loved to know how the church got along there. What problems did they have, and how would Paul have addressed them. Well, I suppose such things will have to wait for when I meet Paul after this life, and perhaps he will have time to sit and explain things over a cup of coffee or tea or whatever they may drink in heaven.