I heard both that the storm door was chained so that it couldn't be opened, and that Amanda was afraid to open it in case he was testing them, so she called for help. It makes me wonder if he'd screwed up before, and there were missed opportunities, or if this was really the first chance they got. I find it hard to believe that this was their first real chance in 10 years, but I do find it easy to believe that they were too afraid to take potential opportunities before this, considering he beat them if they tried to escape.
As for whether he left it unlocked on purpose, I don't know. If he did, I don't think he did it consciously. Subconsciously, maybe.
Considering a lot of kidnapping victims don't even try to escape when they are allowed outside, I am not surprised that these girls were too scared to try previously. Two of them were too scared even when Amanda went for it. They were hiding upstairs and didn't go with her.