There were three major periods in the preparation of the apostles for the work that laid ahead of them: first, they had three-and-a-half years of watching Jesus work; hearing him teach; asking him questions; and, finally, watching him die. This was their first huge preparation period—the three days that Jesus was in the tomb would have been the worst days of their lives. But that all ended with the resurrection.
Then began 40 days of visits by Jesus. They were to be able to ask questions, get answers, and get things clarified that they had not quite had straight before. Here is where they were able to make sense of so many things that Jesus had said and done.
Now, after Jesus’ ascension, they had to somehow attach meaning to everything that had happened. Ten days were not very long, but they could begin to get the bricks into place.