The answer is traffic and availability of the public transport vehicles.
This is based on my personal experience. Sometimes, I won't be able to take my own vehicle on my way to work. And this would leave me no choice but to commute myself to work. I should take a bus or subway or a cab. But on peak hours on weekdays, numbers of commuters like me would struggle to find an available public transport vehicles. Luckily, I could take either bus or a subway. But for others, these might not be available to them. Sometimes, major roads has so much cars in them that would result to extreme traffic. Bumper to bumper traffic are the worst. These are normal in megacities. This could result to waste of energy and time for the commuter.