Why was the B-52's wet wing more susceptible to fatigue than its dry wing, and not less?
Earlier B-52s did not carry all their fuel in the fuselage. They also carried their fuel in the wings using rubber bladder-type wing tanks. 

The new B52G “wet wing” was a weight savings measure which also increased the fuel capacity over the bladder-type in-wing tanks. The metal fatigue of the new wing was due to a poor structural design which was supposed to save weight.  

http://www.joebaugher.com/usaf_bombers/b52_15.html


  The B-52G had a lighter structure than previous versions in order to save weight, but it carried more fuel. This meant that fatigue problems resulting from the structural flexing generated by the stresses of low-level flying and midair refuelling manifested themselves earlier in the life cycle of the airframe. This was especially the case in the region of the wing structure where most of the weight savings had been achieved by using an aluminum alloy. Fatigue cracks got so bad that stringent flying restriction had to be imposed, pending modifications. In May of 1961, a program was approved in which the wings were modified and strengthened as part of the regular IRAN schedules for the aircraft in the B-52 fleet. The project was finally completed in September of 1964.


Early B-52B,C,D fuel chart showing multiple wing tanks.