Two reasons Vader let him go:
1) he sees OWK has help so he wants to see who is helping him. But the main reason is:
2) he wants a PROPER rematch. His ego dictates he wants to beat his old master, OWK is a shell of his former self. DV wants to prove he's the best. If he took him now it wouldn't prove anything, be a hollow victory.
The key reason surely: Vader wants Obi Wan to suffer as he did, entirely on Vader's terms. No quick slice and dice. Plus, now Obi-Wan is trapped on this planet, Vader will find him and do whatever he wishes.
Two reasons Vader let him go:
1) he sees OWK has help so he wants to see who is helping him. But the main reason is:
2) he wants a PROPER rematch. His ego dictates he wants to beat his old master, OWK is a shell of his former self. DV wants to prove he's the best. If he took him now it wouldn't prove anything, be a hollow victory.
Loving it so far, Ewan is great.