An upswing of interest in hiking the entire Wonderland Trail has led to an unprecedented number of permit requests. Not all of the currently received requests will be processed and obviously no new ones. From the Wilderness Camping and Hiking page:
April 1, 2015: THE PARK IS NO LONGER ACCEPTING RESERVATION REQUESTS FOR PARTIES ATTEMPTING TO HIKE THE ENTIRE WONDERLAND TRAIL, OR ANY LARGE PORTION OF IT. We have received a staggering (and record) number of wilderness reservation requests between March 15 and April 1 (2,500 and counting). We will not be able to fulfill all of the requests we have already received, much less those that arrive after March 31. SUMMERLAND and INDIAN BAR will also fill before the March 15-April 1 batch is completely processed. The option to a reservation is to attempt to get a first-come, first-served permit.
There will be about 30% of the permits available, but it's a giant pain in the ass to get them and there are no guarantees. Go this route only if you have patience and live close.
The Seattle Times also has an article on the issue with a little more background.