There are many Mekoros that suggest that one must write down their sins. Among them, Shabbat 12a, Yoma 80a, Tosfos Shevuos 26a, Rama (OC 334:26), etc. Why is it that, seemingly, no one follows this Halacha?
The sources you mention refer to writing down a sin that involves a “sin offering.” Certain Talmudic Sages used to do so, expecting the Mikdash to be speedily rebuilt, and wishing to be reminded of the offering they have to bring.
There is no halachah whereby a person has to do this, and the anecdotes mentioned by the Gemara are on an anecdotal level alone.
The Shaarei Teshuva (1:8) notes that a person should write down his sins in order to repent for them. This is recommended practice, but falls short of a full obligation.