"Unknown Or Inaccessible In The European World"
Rabbi Avraham Chaim Bloomenstiel commenting on "What Works For One Will Not Work For Another":
This discussion of the Meam Loez has overlooked three major points:
First, it is a relatively late work. Rav Culi began compiling it in 1730 yet died before it was completed (he only finished Bereshis and part of Shmos). A number of other rabbonim continued to work on it throughout the 18th Century until it was finished by Rabbis Yitzchok Arguiti and Shmuel Yerushalmi.
Secondly, it was published and disseminated almost entirely in Turkey, North Africa, Spain, and Egypt.
Thirdly, it was written in Ladino - A language that was not spoken by the Ashkenazi world (hence the title "Meam Loez"). The work was not translated into other languages (i.e. Yiddish, and even a modern Spanish printing by the University of Granada in 1964) until after the Holocaust. The book never existed in a Hebrew version until 1967!
For these reasons the work was not well known in the Ashkenazi/Chassidic world until fairly recently - There was no shitto historically to avoid the work; the book was either simply unknown or inaccessible in the European world.
As for Rabbi Kaplan ztz"l's comment about the work being "shunned" - We have to assume that R' Kaplan is only speaking post 1967; after the date of the first Hebrew edition. Before then it was not well known in the Yeshivishe world.
What does R' Kaplan mean by "shunned?" I don't know. These are a lot of historical items that could explain the comment, but many of them may not be true nowadays. At that time the "Canon" of acceptable Yeshiva learning was much more rigid than it is today and it was hard for any "newer" sefer to become part of it (excepting those authored by certain Gadolim in the Yeshivishe world). Or, it could be that there were certain biases harbored in the Ashkenazi world against the Sephardi mesoras of limud Torah. Nowadays these biases have mostly dried up and are outdated. Yet, perhaps because it was a "quintessentially Sephardi," work the Meam Loez may not have held much esteem in the Ashkenzai eyes of the time.
In all truth, these explanations of the "shunned" comment don't get us anywhere productive.
There are actually numerous seforim that you could ask "Why weren't these learned in Yeshivos in the 1960's, 70's and 80's?" Most of the time the issue may just come down to canon; It just wasn't the derekh of the Litvishe yeshivos to learn them. This attitude doesn't speak toward a particular bias or disdain, rather just a "curriculum" issue.