Well really there’s no other time period to set it in. You can’t set it pre-Phantom Menace unless you want the movie to be about kiddie Obi-Wan training, which I don’t think anyone wants to see. You can’t set it in between 1 and 2 because then you’d have to deal with Anakin’s training, effectively creating Prequel 1.5. Don’t think anyone wants to see that either. You can’t set it between 2 and 3 because The Clone Wars already covers that. And you can’t set it any time after 4 for obvious reasons. So between 3 and 4 is pretty much the only time period you could create a decent Obi-Wan story in. Also the only time Ewan is appropriately aged for now.

If they do decide to go the route of making Rey related to Obi-Wan, such a movie could definitely tie into that and likely humanize Obi-Wan and flesh out his character a great deal, seeing as he was portrayed as a stern monk in the prequels without much of a personality.