Q:

Why is the Gravina Island bridge held up as an example of wasteful pork-barrel spending? 

The as-yet-unbuilt Gravina Island bridge would connect the Alaskan city of Ketchikan to an island that holds the city’s airport (plus a few other residents besides); the two are, currently, linked only by ferry.   Due to the very low population of the island coupled with the project’s enthusiastic support from former governor Sarah Palin, the proposed bridge is held up as a prime example of wasteful pork-barrelling, having been dubbed the “Bridge to Nowhere”. 
Looking at it, however, it seems to me as though the presence of the airport would be sufficient justification for the bridge all on its own, even if the island were otherwise completely uninhabited:

Transportation links between cities and their airports are generally considered eminently worthwhile investments, even if they serve nothing else, simply due to the degree by which they ease travel between the city and what, in Alaska, at least, is frequently the city’s main - or even only - link to the outside world at large. 
Secondly, and, perhaps, more importantly, the lack of a bridge seriously degrades safety at the airport, by making it impossible for outside emergency forces to render timely aid in the event of an accident beyond the ARFF units’ ability to handle.   This has actually happened at least once; in 1976, Alaska Airlines Flight 60 crashed on landing, and the Ketchikan ARFF units were unable to effectively fight the fire from the burning aircraft.   The city fire department sent men and machines to help, but, delayed by the need to take the ferry, they did not arrive until 21 minutes after the crash, right as the last of the surviving occupants was being evacuated (fortunately, only one person died, and from impact forces, not fire). 

What am I missing? 

A:

Bridges are expensive.  Some don't like government spending large sums of taxpayer money where they cannot see the value.  To that end, it is necessary to hold up an expensive example with a catchy phrase.  In this case, it was Taxpayers for Common Sense, the Gravina Island bridge, and "Bridge to Nowhere". 
A New York Times Magazine article explains how it came about for the Gravina Island bridge.  Other bridges and other spending are mentioned in the article. 

“Keith Ashdown was a few beers into a night at the Hawk ’n’ Dove,” wrote Tory Newmyer in the Capitol Hill newspaper Roll Call, “when he came up with the phrase that sparked a national debate over Congressional spending.”  Reaching for some way to get people worked up about “earmarks” directing taxes collected by the federal government into obscure local construction projects, the lobbyist for the unassailably named Taxpayers for Common Sense was struck by what he recalled was “a moment of sheer focus.” 
He came up with a moniker for a proposed costly bridge to a sparsely populated island in Alaska.  At first, no reaction; but three years later, the atmosphere became charged with scandals of political influence-peddling, and The Washington Post reported that “the Bridge to Nowhere became a national symbol of porkmania.” 
Ashdown’s Bridge to Nowhere was the phrase that launched a thousand editorials.  On the left, Salon noted with scorn that “Alaska’s Gravina Island (population less than 50) will soon be connected to the megalopolis of Ketchikan (pop.  8,000) by a bridge nearly as long as the Golden Gate.”  On the right, the Heritage Foundation denounced the planned span to the Ketchikan International Airport as “an object of national ridicule and a symbol of fiscal irresponsibility.” 

What am I missing? 

Perhaps, that local needs are not national needs.  That is why earmarks for local projects are regularly attacked as wasteful spending.