Why are medical doctors susceptible to sales pitches by pharmaceutical salesmen/women?

Because doctors are human. If you want to learn more, read up on certain topics in psychology. 90% of drug reps are attractive or attractive+super charismatic. They are foremost sales people who are very skilled at selling their product (although many do have a background in science or healthcare!).

Isn't it a doctor's responsibility to know more than a sales rep?
No. It's a doctor's responsibility to diagnose and treat illnesses. Doctors aren't magical beings who know everything about the thousands of prescription drugs out there or the couple hundred they routinely prescribe. Sometimes, doctors have several drugs they can choose from that do basically the same thing in slightly different ways, with slightly different risks, and slightly different side effects. Say you are faced with a problem---you want to listen to music (can't think of a better analogy). So obviously you need headphones. There are several different types of headphones. There are earbuds which are cheap, comfortable to wear, easily to replace, and many stores even offer discounts on these already cheap headphones--but they don't produce good sound and are considered a little outdated. There are on the ear headphones which produce better sound, some stores offer discounts on them, but they cost more than earbud headphones. Then there are next generation over the ear, noise-canceling headphones. These produce superior sound quality, but (for the sake of analogy), they are very uncomfortable to wear, very expensive, and almost no stores offer of discounts on this type headphones. Now the a guy from Sony, visits you at work, and buys the whole department lunch in exchange for you listening to a spiel about Sony's new on the ear headphones. He hopes that you will recommend Sony's over the ear headphones to your customers. Are Sony's over the ear headphones "better" than the earbuds or noise canceling headphones offered by its competitors. Not necessarily. But what if a customer is allergic to a material in a different company's earbuds. Or what if the noise canceling headphones are considered overkill for a customer that just wants headphones for his daughter's ipod? What if it doesn't matter what headphones a customer gets and Sony is the first to pop into to your mind because they bought you food? This is how drug sales work.