From the interviews, what I have gathered is they wanted Bradley for Arthur from the beginning because one of the producers worked on DisConnected. They were probably ecstatic with his chemistry with Colin. They cast for Gwen the last for some unknowable reason. Probably figured that both Bradley and Angel were charming individually they would be even more awesome together. Realized throughout season 1 that they had no real A/G scenes, realized in season 2 that even with scenes they have no chemistry, and in season 3 became obsessed with the A/G to the detriment of everything else except evil Morgana. I guess given that they're so determined to hold out on the magic reveal, they need to write about something and thought, "Why don't we fix A/G?"