Those listed as favoring reversal--that is, those who favor including the wetlands in question within the section 404 permitting scheme--include one environmental group and seventeen state governments.  It looks as if the positions of these groups are represented in just two briefs.  One is a brief of the environmental group (the National Wildlife Federation)  in conjunction with the state of Michigan (where the property in question is located); the other is a brief joined by sixteen state governments.

You're probably not surprised that the National Wildlife Federation favors reversal.   Environmental groups typically feel that the federal government will do a better job than local governments in protecting environmental interests.

Why would state governments favor including more wetlands within the section 404 scheme?

One possibility is implicit in the first sentence of the first numbered footnote in the opinion: "With respect to certain waters, the Corps’ authority may be transferred to States that have devised federally approved permit programs."   More land within the federal section 404 scheme is therefore more land potentially within the State government's regulatory authority.  If state governments want more power, then it makes sense for state governments to favor reversal.

Another rationale assumes that state governments might actually want less power, because less power makes for a lessened burden of governance.  "Heavy is the head that wears the crown," as the saying goes, and the state governments might want the federal governments to wear the wetlands-permit crown.  That way, the federal government will take the political heat from local land developers or farmers when a permit is denied, as well as taking the political heat from the environmental groups when a permit is granted.  Federal rather than state administration of the program also presumably makes for lower monetary outlays by the states, perhaps allowing the state government to lower politically unpopular taxes.  If state governments want more wetlands at the lowest possible "price" to them, then it makes sense for state governments to favor reversal.  (Note that this rationale is most potent if the state governments can count on the federal government to make the "right" permitting choices--that is, the choices that the states would make themselves.)

It's also tempting to look down the list of states to see if one can discern any particular pattern.  Here they are:

This list includes some states that one would imagine in advance would be pro-wetlands.   There are, after all, some states on the list that promote tourism by emphasizing the natural beauty of their lands and waters, such as Hawaii and (at least for the Outer Banks) North Carolina.  There are states with lots of wetlands along their coasts (Connecticut, Maryland, and Rhode Island) as well as further inland (Louisiana and its bayoux).  There are Western states (California and New Mexico), which might see wetlands not so much as an environmental issue in the classic sense but as one of a set of weighty concerns relating to water supplies and flood control; they might also just generally favor whatever position the Corps takes in order to build up political capital with an agency that has a lot of impact on their particular states.

But note also the presence of Midwestern farm states (Illinois, Missouri, Nebraska, and Wisconsin), where one might expect the state government to oppose federal regulation of wetlands for fear of offending the ever-powerful farm bloc.  And West Virginia, which one doesn't typically consider an aquatic paradise, is also on the list.  So are Vermont and Tennessee, also known more for their mountainous venues and certainly not known for their coastlines.  Montanans tend to praise Big Sky and Small Government, but even their attorney general weighs in here in to favor federal regulation.  And California is so large in population (roughly 1 in 9 Americans live there) and diverse in biomes (redwood forests, Death Valley, the Sierras, irrigated croplands, conurbations) that it's practically a coalition of states all by itself. 

So you have nearly one-third of all states represented, with a scattering of states from each major region.  There are tourism states, water-obsessed states, states with lots of wetlands, states with lots of mountains, states with lots of farmers, and Montana.

Notice also that no state filed an amicus brief urging affirmance.  The fact that it's 16-0 (17-0 if you count Michigan) in favor of reversal presumably also tends to show that states have broadly similar interests in federal regulation of wetlands.

A digression:  Fans of American political history will see at least two famous family names in Midwestern politics represented in the list of attorneys general: Hubert H. Humphrey III in Minnesota, and Bronson C. La Follette in Wisconsin.  (You may also note that West Virginia's attorney general has apparently graduated from a life of treed kites and missed football kicks to the life of the law.)