Science (supposedly) says. At the link, summaries of studies that show the correlation.
Now, why is there a correlation? Let's brainstorm...
1. Drinking is a way to dial down your intelligence, thus adjusting yourself to the world as it is, built around the needs and interests of average people.
2. You'll still be able to get your tasks done. In fact, maybe you've been zipping through your work too quickly. If you were dumber, you couldn't afford to hamper yourself with drink.
3. Being smart, you find it easier to locate drinks. At some point, you'll be saying "Where's my drink?" like the next guy, but you'll have that extra margin of drink-locating capacity.
4. (An idea from the article.) "[T]he human brain has trouble dealing with situations that did not exist in the Pleistocene environment we evolved in... 'general intelligence evolved as a domain-specific psychological adaptation to solve evolutionarily novel problems.' Alcohol consumption is 'evolutionarily novel'—humans began cultivating and consuming alcohol only about 10,000 years ago," so smarter people are better at drinking.
Surely, there are more ideas here.