Why is the book of Acts called The Acts of the Apostles?

There are two schools of thought on this. The first is that an "Apostle" is somebody that personally saw Jesus after the resurrection, and was then a witness to that fact. This school of thought claims that there are no more apostles, other than the 12 + Paul. Because you are asking this question, I assume you are from the first school of the thought.

The second school of thought is that an apostle is what the Word literally means, one sent forth with a message. In this case, they were sent forth with the gospel message. That is why in Matthew 10:1-7, in verse 1, Jesus called his 12 disciples(followers) to himself, and then sent them away as his 12 apostles (sent messengers) in verse 2. Barnabus was a sent messenger (Apostle) to spread the gospel along with Paul.

There is a lot more to say on this subject, please let me know if you need more elaboration.