It depends on which group of Mustang enthusiasts you ask. Not all Mustang people look at II's in a negative way, but of the ones who do, here are few reasons why. For the non II owners reading this post, please excuse the satire.
It's hard to grasp, until you've logged a few miles in a II.
1) The Original Classic Mustang owners (including those that bought them years later) as potential investments:
This group of original owners could never accept the idea that Ford created a Mustang without a V8 option. In their minds, the II was/is nothing more than a glorified Pinto.
Many in this group feel the II is an insult to the heritage of the "original car", in spite of it receiving Motor Trends 1974 car of the year award due to it's all new design and truly technological improvements.
2) Fox Owners...the 5.0 crowd:
This group carried on what (by then) had become the popular thing to do, (ignore and snub II's) and probably in part to shift the focus away from the fact that their own cars didn't really look like traditional Mustangs. A rather negative group at times, they are sometimes known to dislike their own siblings...(aka '79-'81 models) for similar reasons.
3) SN95 Owners:
With the introduction the all new models of '94, Mustang II bashing was soaring to new heights and thirty years running. This group was sure that with the return of the signature C scallops on the doors, Ford had reintroduced a true Mustang that captured the heart and soul of the originals, while at the same time correcting all the shortcomings of Ford's previous attempt a decade before...the Mustang II.
4) SN197 Owners...the '05 RETRO crowd:
Forty years after the fact, a new generation of first time Mustang owners and previous classic owners that are attempting to relive their youth, are drawn to an all new Mustang.... that while being a retro copy in many ways to the classics, also has some uncanny, subtle, yet undeniable similarities to the Mustang II. Few of the owners in this group will admit it, and many of the rest are too young to realize it anyway.
Media sources touting this lastest Mustang as "the best ever" (don't they do that every year?) will have this group on an ego trip for the rest of the decade. It's filled with two distinct core groups of enthusuasts (separated by age) that either never liked a II, or were indoctrinated through poor parenting skills to believe II's were the not "real Mustangs".
So, there you have it in a nutshell...a happy family of Mustang owners that either dislike, can't stand, or accept the thought of such a skeleton buried in the heritage of their Mustang's past.