I will preface my question with a brief summary of the sugya as I understand it. The debate on whether one should wear Tefilin on Chol HaMoed seems to have both a סברא component and a textual component.
The סברא component is whether Chol HaMoed is considered an אות. Tefilin are considered an אות and are not worn when the day itself is also considered an אות. Shabbos and Yom Tov are considered an אות because of the work prohibitions and therefore Tefilin is not worn. The question is whether the Mitzvos of Lulav, sukkah, matzah, and partial work prohibition on Chol HaMoed also makes it an אות.
The textual debate is between the simple understanding of a Mishnah and the explicit understanding of the Zohar. The Mishnah in Moed Katan says that one can write Tefilin on Chol HaMoed. The simple understanding (though not necessarily the only one) is that is permitted to write because tefilin have an immediate use… There is a requirement to wear them on Chol HaMoed. The Zohar however is explicit that one should not wear Tefilin on Chol HaMoed… And the Zohar speaks very strongly against this practice.
It seems that this dispute falls along “party lines”. Chasidim and Sepahrdi Achronim who normally put more emphasis on sources like the Zohar, say not to wear Tefilin on Chol HaMoed. Most early Ashkenazic Achronim/poskim who normally do not emphasize the Zohar especially in determining halacha, say to wear Tefilin on Chol HaMoed.
The one outlier is the Vilna Gaon who said not to wear Tefilin on Chol HaMoed (which is the reason why most Orthodox Jews today, including Ashkenazim, especially in Israel, do not wear Tefilin on Chol HaMoed). However, I do not know why the Vilna Gaon ruled like that. Did he put halachic emphasis on this Zohar? Did he have reason to learn the Mishnah in Moed Katan differently? Why did the Vilna Gaon divert from most of the other Ashkenazic authorities?