Easy. Good habits require work, plain and simple. Self destructive habits on the other hand are generally rooted to a mental addiction. Your bad habits are much easier to maintain because they trigger dopamine receptors, nature's motivational hormone.
Perfect examples are junk food or pornography. Instinctively, there are basic decisions you will make subconsciously for survival purposes. One of these includes ingesting as many calories as possible in one sitting (because in a survival situation you never know when you might get another meal). When you eat highly caloric foods like junk food, it triggers dopamine receptors in your brain to give you a rewarding feeling. Evolution hasn't had a chance to catch up with the secured lifestyles many of us live, and this basic instinct becomes a self-destructive habit when high caloric foods are always plentiful.
As for pornography, you have another basic survival instinct to want to mate with as many different women as possible(to increase breading chances). The vast collection the internet offers lets you have a different woman every couple of seconds and you brain doesn't distinguish between finding a real mate and illuminated pixels on a screen that look like one. Every time you find one, dopamine receptors are triggered for that same rewarding feeling compelling you to continue. Like junk food, evolution hasn't had a chance to catch up with the overpopulated world we live in today.
These are just some examples. Many self-destructive habits are rooted at the exact same problem. You want to do these things you know are bad because your body is physically telling you that you want to do them. It takes a lot of willpower to consciously overcome such instictual, natural responses.
This is a pretty good question I must say.
Giving it some thought, destructive habits, at least ones dealing with the self and how you take care of yourself require little to no effort to accomplish. Safe for example not showering, you don't actually have to do anything at all to accomplish that. It's the lack of productivity that leads them to be more feasible to do than their counterparts.
Take for example what you do, which is jogging I presume. You jogged short distances and eventually built up to several miles but that took a lot of elements like dedication, perseverance, and effort just to maintain it. To exceed you need to do that and a bit more.
The reason I feel that it's easy to fall into a self-destructive pattern is because sometimes, we get tired of cyclic routines that seem to continue each and every day at some point in our lives. I can't say in anyway that this is factual, it's just what I believe at the present moment. Eventually, however, people just drop whatever cycle there is to their day and adopt something else and this something else is usually a lazy self-destructive lifestyle and become adapted to that.
That's just what I think though. Anyway, awesome question, really got me thinking :)
I'ts the easier way out to stay in the bad then actually make effort to make yourself happy and stay that way. Your the only person who can make yourself truly happy and not many people grasp that.