פירוש על ברכות 39:3

No analogy can be drawn between the wearing of mixed stuff and the burial of a Met Miswah, because they fall into different categories. The former is a prohibition as an act — one has to "arise and do" [קום ועשה] it (by putting on the garment), to commit the transgression. With the Met Miswah, on the other hand, it is the omission to bury which constitutes the transgression. To "sit and do nothing " [שב ולא תעשה] is what is forbidden.