Shalom Samuel,

It's great to hear from you! It is not always clear, to me anyway, why several large portions of Shas do not have Tosfos. As you might know, Rabbi Yissachar Ber Eylenburg, the author of the Sefer Be'er Sheva (who lived around 400 years ago), took upon himself to write commentary on portions of Shas that do not have Tosfos. In his introduction, he identifies several large such sections throughout Shas, including: Horayos, Tamid, the first and last chapters of Kerisus, the last two chapters of Sotah, and -- as you astutely discovered -- Perek Chelek in Sanhedrin.

There are also many Amudim in Maseches Berachos and Maseches Shabbos that do not have any Tosfos.

By the way, there are also cases where we do apparently have a commentary of Tosfos, but it is of the nonstandard form. For example, in the beginning of Maseches Avodah Zarah, the Gilyon ha'Shas cites a Chida who in turn cites the Shitah Mekubetzes saying that the Tosfos which appear on the Dapim of Maseches Avodah Zarah are not the standard, but rather from Rabeinu Peretz.

It is also not clear to me whether in each case the standard comments of the Ba'alei ha'Tosfos were in fact written but just not preserved, or perhaps in come cases it is more likely that for some reason they were not written at all.

I can offer two potential suggestions regarding why no standard commentary might have been written by Tosfos. But I cannot say that both necessarily apply in any specific case.

1) Often, the agenda of the standard Tosfos is to raise and resolve conflicts in the internal logic or legal ramifications of a Sugya. But if the focus of the Sugya is Agadeta, then there may be less need and interest to invest in the Tosfos-style of analysis there. I think this reason might apply to much of the material in Chelek, as well as the sections of Shabbos and Berachos that I mentioned above.

2) Many times, the concern of Tosfos is how to reconcile the local Sugya with other related Gemaras throughout Shas. But if a Sugya is largely unrelated to the material in other Masechtos, then there would be less of a reason to do this. I think this would apply to much of the material in Maseches Horayos.

May you continue to attain greatness in Torah and Yir'as Shamayim!

Warmly,

Yishai Rasowsky