While I am by no means an expert on the subject, I have been researching Techeiles for about 10 years now.
I have come up with 2 major reasons why a significant segment of our population do not hold of Techeiles:
1) There is a Tosefta in the back of Masechet Menachos that seems to imply that anything other than Techeiles or Lavan on the Tzitzit pasuls the entire Tzitzis/Beged. In other words anything on the Tzitzis which is not true Techeiles would be (in my humble understanding) the same as placing Klai Ilan (which we know from Chazal as forbidden). So many may be Chosesh for this stringency.
2) Something that resurfaces after a significant Hefsek in time is considered suspect since the mesorah that deals with the issue was not used/applied in the intervening time. It’s almost a similar issue with eating turkey. Some families do eat turkey and some families do not. Why? Those families that do not eat turkey may simply have had no mesora from earlier generations to compare the simanim of kosher birds to the turkey, and apply it.
I too have observed that those who wear Techeiles seem to be more Mizrachi/Eretz Yisroel oriened whereas those that do not are Agudas Yisroel oriented.
What is also interesting to note is that Klai Ilan is mostly the vegetable dye called Indigo or 2,2′-Bis(2,3-dihydro-3-oxoindolyliden). The extract from the gastropod mollusk called Murex Trunculus is a dye (which many call Techeiles) is actually a compound containing 6,6-dibromoindigo, somewhat similar to Tyrian Purple.