They might not taste very good
The taste buds of children are much more sensitive than those of an adult. As a consequence, the strong (and for a child, foreign), flavors of vegetables can taste particularly bitter. This problem is compounded by the fact that many flavors are new to children when they are young, and it can take five or ten experiences of a new taste to acclimate the taste buds to it.
When was the last time you saw a television commercial (or any advertisement, for that matter) designed to get your kids to eat more fruits and vegetables? Wouldn't it be surprising to see Britney Spears singing an adapted rendition of one of her hit songs espousing the virtues of sugar snap peas instead of Pepsi? Or Shaquille O'Neal hawking baby carrots dipped in hummus instead of Whoppers? Kids are subjected to a daily barrage of marketing tactics from clever marketers convincing them that certain foods are cool, hip, tasty, fun, etc., often communicated by sports stars and other celebrities. These foods do not often include vegetables (with the exception of french fries, the most commonly consumed form of vegetable in the United States).
In response, the government does spend money on advertising to advocate healthy eating, but a recent analysis showed that the entire yearly U.S. budget for such advertising is less than the marketing budget for Altoids mints. No wonder a Dorito seems more exciting than a Brussels sprout.
Lower availability and convenience
Eating vegetables requires effort. First you have to find them. Then they must be cleaned, peeled, cut, and often cooked. You don't typically find them in convenience stores, vending machines, or mini-marts. They are not typically packaged in snack-sized packages, or in roll-up form (like so-called "fruit" roll-ups). By contrast, candy, chips, and soda are ubiquitous in our society, immediately available, ready-to-eat, and portable.
So, with these formidable forces arrayed against you in the timeless struggle to get kids to eat their veggies, what can you do about it? In fact, I have seen many parents win this battle by adopting some simple, practical tactics:
1. Find ways to put the kids in charge of their vegetables. Involve them in the process of choosing, shopping for, and preparing the vegetables. Kids are much more likely to feel good about eating veggies if they think it is their idea. If you can avoid it, don't force vegetables on your kids. It is often a good idea to have them available on the dinner table on a plate at the center of the table, where family members can help themselves, rather than served on individual plates. This way your child can help himself/herself and also mirror the behavior of adults who are helping themselves.