Why are emergency water landings the majority of the pre-flight safety briefing if they're so rare? According to an article published in the New York Daily News a few months after the "Miracle on the Hudson", when US Airways flight 1549 safely landed (watered?) on the Hudson River to the survival of all 155 souls on board, this incident was only the second ever emergency water landing that did not result in significant injuries and casualties in commercial aviation history.

And yet, the safety briefing at the beginning of every commercial flight adhering to aviation regulations spends more time discussing water landings than nearly any other topic, focussing on how to use life jackets, where to find them, where to inflate and where not to inflate them, how to use the emergency rafts, and so on.

Further, a very large amount of weight on commercial aircraft is devoted to equipment chiefly or entirely present to aid in water landings, with most commercial aircraft today having four or six emergency life rafts, hundreds of life jackets, and even the design of the aircraft itself adhering to minimum float time requirements.

With water landings so rarely attempted and even more rarely successful, why is there so much emphasis put on the current, ineffective methods? In other words, if water landings with current methods and technology generally don't work, why hasn't this changed?
The entry doors to an aircraft that is required to have life rafts are equipped with "slide rafts," which are dual purpose (land/water) so there is not any material difference in the weight. Some aircraft will also have perhaps one (maybe two) rafts in the cabin overhead (depending on the need for extra life raft capacity not covered in the entry door slide/raft).

I imagine, if controlled water landings were more frequent, design and technology might be modified in some way to make successful evacuations more efficient if that were necessary.  But, as you point out, controlled water landings are relatively rare, so likely the cost/benefit of creating an entirely different fuselage/wings/evac system would not be justified.

Of course, the fairly recent successful controlled water landing and evacuation into the Hudson is ample justification for having personal flotation devices (life vests, etc.) and associated emergency equipment designed primarily for water survival. Also, the design of that aircraft apparently provided for adequate flotation and evacuation as can be seen from the photographs.   

The safety briefing before each flight by the Flight Attendants is relatively short and maybe 60 seconds are dedicated to explaining the operation of emergency equipment specific to water landings.