There are probably several reason why.
First, not everyone has an easy time making the transition from smoking to vaping. Some folks have dual used for months and months. Some were successful, others were not and returned to just smoking.
Vaping doesn't work for everyone, although my guess is it will work if you are persistant in your efforts and very determined to quit smoking. Recent studies claim that vaping is twice as likely to help smokers quit compared to using nicotine gum.
Vaping is not like smoking. It's different. Vaping is less efficient in delivering nicotine to the body, so one needs to vape more frequently than they smoked.
Differences Between Smoking and Vaping
I had smoked for over 30 years. I loved smoking, and had begun to believe that I'd never be able to quit. But it was beginning to affect my health. Over the last 10 years I had tried nicotine gum, patches, prescription meds, and even hypnosis. They were all very unsuccessful for me.
I decided to give vaping a shot. I bought a ciga-like starter setup, had one pack of cigarettes, and promised myself that I wouldn't buy another pack. I knew that I would have strong cravings for a smoke, so I decided to dual use at first but to limit the amount of cigarettes that I would smoke.
When I had a bad craving, I vaped more. If that didn't help, I allowed myself HALF of a cigarette and saved the last half for the next bad craving. Probably smoked four whole cigarettes the first couple of days. Those unfinished cigarettes tasted pretty nasty, but that helped me quit. By the time that last pack was finished, tobacco was beginning to taste pretty nasty to me and I didn't feel the need to buy another pack. This method worked for me, but it was still tough. I was DETERMINED that this time I would finally quit smoking.
If you choose to dual use, switch cigarette brands. Avoid your favorite. If you smoke menthol, use a non-menthol. If you smoke non-menthol, use a menthol.
Avoid situations that you know you'll be tempted to smoke. Bars or parties. Other smokers. Alcohol adds to your cravings. Stay busy. Avoid stressful situations when possible.
Why You Don't Need an E-Liquid that Tastes Like Cigarettes
A Word of Vaping Technique
Finally, don't beat yourself up should you fall off of the wagon. Smoking is an extremely strong addiction and we have human weaknesses. If you fail, pull yourself up off of the ground and start over. It's not how many cigarettes you smoke a day that is important...it's how many you didn't smoke that is.