IMO it’s more collectors who can’t bear to miss an issue because it would put a hole in their long running collection.
Also, these charts don’t really tell us anything but ranking in 125 stores. Without actual numbers, we don’t really know how well something is selling.
However, it appears those who don’t like the current ASM are buying USM. The monthly chart doesn’t have the ratios but the weekly top ten charts do. And on the first weekend of release, USM is outselling ASM by more than 2 issues to 1.