The Metrorail stop is a mile away from the waterfront and is a much longer haul from DC than, say, Clarendon.
There are plenty of arguments about all kinds of locations in DC that are a pain ********** to get to. Georgetown being one. If anything, being difficult to get to generally has the benefit of keeping the city riff raff out of the area (usually).
I'm sure you heard the old rumor the reason there is no Georgetown metro is to keep the homeless away.