Paul gives an explanation here about why he could not find anyone else. All (of his other fellow workers) are not suitable. They all seek their own interests. This does not necessarily mean that they only have eyes for their personal affairs, for it is difficult to say what exactly is meant here with own interests.
Paul honestly expresses that most of his fellow workers are not willing to put everything aside for a difficult mission. The work of Christ is not all-consuming for them. Compare this with Philippians 2:4, where Paul reminds the Philippians this way: Let each of you look not only to his own interests, but also to the interests of others.
Whoever is still doing that is not unfaithful to Christ, but yet not suitable for an important mission task. That is the reality with which Paul is confronted, and for this reason, he can do nothing else but send his closest fellow worker Timothy. In Philippians 2:20–21 it becomes clear that serving Jesus Christ (Philippians 2:21) and serving the church (Philippians 2:20) go hand in hand. They belong together!
21 For they all seek their own interests, not those of Jesus Christ.