I think it's because -
1) people just don't want to make the commitment (it's all about convenience)
2) its "natural" for them to eat animal flesh (blech). As yucky as it is to admit it, most ppl are raised on meat.
3) most are also ignorant (i.e. they think that veg alternatives are gross even tho they never tried them and fail to realize how many "normal" foods are vegetarian).
4) most never even consider it for some reason. For instance, when I was in HS the thought to become vegetarian never even crossed my mind. It wasn't until I started researching my food choices that I decided that this was the way to go for me.
And I'm sooo glad I did eventually go veg