Hello Matthew,
First of all, there's not just 1, there are hundreds of meaningful reasons to quit smoking, and every smoker knows them, I'm sure you do too. But all smokers choose to ignore those reasons so that they can continue smoking.
I quit and failed many, many times in the past 40 years. The willpower slowly faded away and the many great reasons got replaced by pathetic excuses as the "cravings" get stronger and stronger.
The reason you're having a hard time quitting smoking is not that you're lacking willpower or you don't have a good meaningful reason to quit. The reason for failing is the lack of understanding what smoking is all about. It's not just a bad habit, as Doreen already indicated: it's an addiction
People don't smoke because they enjoy it, people smoke because they're addicted to Nicotine and they need to feed the addiction, period.
For me personally, educated myself is the reason for my successful quit so far. Once I learned about the addiction, I was able to overcome all the cravings (physically and mentally) without relying on "willpower".
My only advice is educate yourself about the Nicotine addiction, there are many great materials on this site in the static posts as well as links to other great resources. The understanding about this nasty addiction will enable you to quit much easier than relying on "willpower".
Start with the post on "How Nicotine works", then slowly go through all the other materials, the more you read, the better understanding you have, the easier it is to quit.
Welcome to Quittrain, hope you stay and enjoy the ride.