I've found many reasons:
1. The decision was made for most people when they were babies. They grow up this way and it would be hard for them to stop
2. They've been trained that it's healthy, that you don't get enough vitamins from a meat free diet. They are not educated on the fact that most people can be just as, or more healthy, without meat. There is a lot of research from reputable mediacl research labs, including Harvard U., that suggest this. You don't hear much about it because the meat industry is very powerful.
3. They have it in their heads that soy and tofu are gross.
4. They don't want to put the required effort into finding new foods to eat, adjusting the way they eat out, cooking different things
5. Food is a social activity and fear not being able to dine out with friends, awkward moments at a dinner party, or getting teased.
6. They don't realize (or do not want to know about) how the piece of meat got to their table. I know many people that, if they were required to kill their own animals for dinner, would become vegetarian really fast.
7. It tastes good. They can't imagine living without it. I though this for a while until it clicked that even though a steak tastes good, it's not good enough when I think how the cow was treated. There are alternatives that are 99% as good and make me feel infinitely better about enjoying it.
These are all reasons why I didn't become a vegetarian sooner, and why many people who I am close to tell me they won't become vegetarian. I think these are pretty fair assessments
I have just become a vegetarian, verging on vegan. I couldn't be happier. I just always remember not to be judgemental, I never try to convert anyone, and don't make my vegetariansim interfere in the rest of my life. When people are interested, I appreciate it and am very happy to share the research I put into giving up meat.
And to the person who mentioned leather shoes- Doing something is better that nothing. That would be like saying that a person who gives $5 to a worthy charity shouldn't even bother since it's not enough on its own to make a difference.