Partially because it's unhealthy, and also because it's addictive. We are advised to avoid anything that is addictive.

I drink Caffeine-free soda, and that's fine. I sometimes drink regular soda, too, and I don't feel bad about it. Like Rxy said, no one is going to come and kick you out of the church for having a Pepsi or even an occasional drink (guilty as charged). It's more a matter of moderation.

A good way of thinking of the Word of Wisdom is explaining things to the lowest common denominator. Most people can handle having a soda, or a beer from time to time without becoming addicted or letting it control their bodies/lives. But some people can't, or don't realize that they shouldn't, so the Words of Wisdom advises against it. It's kind of like making saying "Stoves are hot, don't touch them". 99% of the population can operate a stove without burning themselves, but you've got to lay it out for the 1% who don't get it.

Does that make better sense?