Because the article about it specifically mentions that this is the failure mode to avoid:

Norman R. F. Maier noted that when a group faces a problem, the natural tendency of its members is to propose possible solutions as they begin to discuss the problem. Consequently, the group interaction focuses on the merits and problems of the proposed solutions, people become emotionally attached to the ones they have suggested, and superior solutions are not suggested. Maier enacted an edict to enhance group problem solving: "Do not propose solutions until the problem has been discussed as thoroughly as possible without suggesting any."

So "hold off on proposing solutions" is just one possible solution. Deciding to take that solution immediately, without considering other options (such as NGT's approach) is precisely falling into that same trap.

In short, hold off on proposing the solution of "hold off on proposing solutions". v(^.^)v

edit:

Consider that under NGT, you are given 10 to 15 minutes to think of solutions before anyone gets to propose any solutions. That strikes me as longer than a typical "hold off".