Red, imo
1) the alleyway was dark and rather creepy to go along at night, especially for women checking
2) The back of the flats was a dark quiet place to leave a door unlocked and as you correctly say the patio door had no locking from the outside.. It would have ben vulnerable to burglars.
3) the Mccanns didn't have to go along any dark alleyways, and the rear entrance was well lit by the street lamp on the pavement opposite. Their rear entrance was basically on the street. Burglars would have to be crazy to consider going in that way when it was so well lit and overlooked by peeps in the tapas area.
4) It was by far the most convenient route for the Mccanns. Their walk to the front door was further than ony of the others.
5) For the Mccanns the front door wasn't a "nice" route, especially for womeen. The doorway was deeply recessed, out of sight and it was in pitch blackness. I feel sure it was reported that the porch light was broken. Pls correct me if i am wrong
6) I have a feeling that Matt used the back patio door to his apartment the once, but I haven't checked the rogs so pls correct me if I am wrong. Certainly on the Mccann reconstruction he appears from the direction of the alleyway.
Hope this sorts it out for you. there are probably other reasons that i haven't yet picked up upon.