Very good question, indeed. In vitro studies show that yeasts thrive on a more alkaline environment. But, as the previous reply says, our gut (the yeast harbor) is more acidic. Actually the good flora promotes acidity (hence the name of the major actor "Lactobacilus Acidophilus") and this is true all along the small intestine, and ph goes even more acidic in the colon.
So when people say you should be more alkaline, they basically refer to your blood and intracellular ph, all around the body, but specially the blood (although blood ph levels are controlled by the body and are very stable, some researches found that small deviations have a huge impact on many health issues, like cancer and Candida for instance). But yeasts in the blood are not that much of a problem since only the spores can survive there (well they can become a problem if the colonize some body part)). That's why it is so important to focus on your gut first. Good stomach acids and ph lowering probiotics, etc.