As Moonsword points out, every author has a difrent idea of what Mech of the Week (or whatever) means to them. For somes, its a chance to take a look at new and underutalized mechs as a way to give them a bit of life; for others, its a chance to examine the classics, mechs every one of us knows and has used or faced at least once if we've got more than half a dozen games under out belts, to draw out that common experiance and explore what makes such mechs to popular. For others, its just about requests.
So, when you don't see a given mech up, its not because none of the authors like that mech; quite the conterary, it may be because many of them like it very much, and feel that it will be so well known as not to need an introduction (likely the case for many authors and the Timber Wolf/Mad Cat).
It also may (in both cases at hand) simply be that some mechs offer a truely hurculean task to properly cover. My Marauder article ran something like 5000+ words, and there were still things I found I would have liked to add in after it had been reviewed a bit (and that was before the addition of several new veriants). Something with ten or more difrent incarnations like the Timby or Beemer would require hours of reserch, writing and review/editing (yes, despite my nortoriously bad spelling when posting on the quick, I did tend to try and check my articles as best I could). Even a fairly cursory, slapdash job would still take the better part of an hour, in my mind, if it was anything more than a cut and past from something else. So all but the most dedicated authors are often aprehensive (and even some of them, too).
But, of course, the glory of our present system is that you can make a request and have confidance that it will be honored sooner rather than later (difrent authors in the one-author days had difrent views on requests). And, as often as not, a request for a repost of a mech with an old article will generate one (I don't think I ever did either, so its not me, in this case).