For me, it's a motivation issue. I had to go to work and had to cycle/walk there. Now, going out in the morning is optional. If I stayed up too late or if the weather is bad (it's almost always bad in Scotland) I'll make an excuse and not go out. After doing it a few times, I broke my morning habit and it's extremely difficult to force myself to go out.
I know it doesn't really make sense but that just adds to frustration. I hate that I cannot force myself to do daily exercise and it makes me feel like shit, creating a sort of feedback loop. I don't go out and do exercise because I feel like shit and I feel more shit because I don't exercise.