why everyone holds we can darshen semuchim in sefer devarim

The gemara often debates whether semuchim can be used as a tool of derash (i.e. using the juxtaposition of two topics as proof to a halacha), but even R’ Yehudah who ordinarily rejects semuchim agrees to its use in Sefer Devarim. Why is Sefer Devarim unique? Rava”n (cited in the gloss of R”B Ranchburg Yevamos 4) explains that the other seforim of Torah are written as dictated by Hashem to Moshe, as indicated by the constant repetition of “VaYidaber Hashem el Moshe leimor…” and were never arranged in a formal structure, e.g. “ain mukdam u’meuchar baTorah”, we know the Torah does not follow a chronological arrangement. Sefer Devorim is written as a speech delivered by Moshe to Bnei Yisrael without explicit reference to being dictated by G-d. Since the Sefer is presented as a human speech, we can assume it follows a specific structure and the juxtaposition of ideas is significant.

The Maharatz Chiyus (comment to Brachos 21) offers a different explanation. The gemara (Gittin 60) debates whether the Torah was given “megillah megillah”, section by section, or “chasumah” as one unit. Assuming the Torah was presented section by section, there is no logic behind the juxtaposition of ideas other than that’s the order they were received. In other words, similar to the Ra’avan’s approach, there seforim of chumash were never redacted and organized in a way that would give the work as a whole structure. Sefer Devorim, on the other hand, was given "chasumah", in its entirety at one time, in the 40th year just before Moshe’s death. Since the entire Sefer was presented as one unit and not in smaller subsections, it must have a coherence as a whole and the juxtaposition of ideas must make sense.

Much is written in Chassidic seforim about Sefer Devarim serving as a bridge between Torah sheb’ksav and Torah sheba’al peh (see Marty Bluke’s post here) based on it being written as coming from Moshe’s mouth and not G-d’s The Shem m’Shmuel writes that the Torah sheb’skav contains within it the seed of torah sheba’al peh, but the roots of the halachos are hidden in hester until Chazal reveal them through derashos. In Sefer Devarim those roots are more revealed than other seforim, it’s torah sheba’al peh aspect is more evident, and therefore even though the idea of semuchim is too enveloped in hester to be used in other seforim, it can be used in Sefer Devarim.