It seems odd that something like the tradition for techeles could have been lost. It seems unlikely that Jews just stopped using techeles at some point and then immediately forgot. Are there no records of how the dye was made?
I watched in this torah live video (https://torahlive.com/courses/tzitzis/) that in the times of the Romans they threatened to put in jail or kill anyone making or transporting techeiles because they wanted it for royal use only. It also says in Menachos (.מג) that ר'אחאי had techeiles, and he passed away in the year 510ce. Then in Midrash Tanchuma it laments the loss of techeiles so we see that techeiles was lost sometime between the years 510 and 800.
It was still around in the fifth century because the Gemorah (Menochos 43A) when discussing how to make Techelis mentions Rav Achai having Techilis sent to him.
It is unclear when and how Techlis got lost but apparently the Roman government did not allow it to be worn because the Gemara (Sanhedrin 12A) makes reference to an unsuccessful attempt to smuggle techeles from Eretz Yisroel to Bavel. The smugglers were almost killed.
Even in the best of times it wasn't easy to obtain. The fish it comes from only surfaced once in seventy years. Presumably as time went on it became more and more difficult to obtain until it became lost altogether.