As a former ecommerce guy I know that there are different levels of Google searches. Just searching for "Bitcoin" is the most generic search that someone just beginning their search would search for. Probably because they heard about the Bitcoin price on the news.
As a seller, you target long tail keywords like "buy bitcoin" or "bitcoin exchanges" etc. Those are the people ready to buy.
So you would only expect a Google trend when it's going parabolic. That's when the dumb money gets in and that's when the smart money sells.
Exactly. Once these new people search "bitcoin" one time to get an idea of what it is, they have no reason to search for it again, hence the search numbers for "bitcoin" drop.