1. It may be that Rashi does not give the reason of Somech Ge'ulah l'Tefilah because not all opinions agree that at night there is an obligation to be Somech Ge'ulah l'Tefilah. The Gemara below (4b) cites a dispute between Rebbi Yochanan and Rebbi Yehoshua ben Levi. According to Rebbi Yochanan, one is required to thank Hash-m for the Ge'ulah before Shemoneh Esreh at night also, but according to Rebbi Yehoshua ben Levi this is required only during the daytime. Therefore, Rashi prefers to give a reason that is valid according to everyone.

2. In addition, not only in the Gemara is there a dispute on this matter, but the Rishonim also debate it. Tosfos (4b, DH d'Amar) writes that according to the Sidur of Rav Amram Ga'on, the reason why we say Kadish at night between Ge'ulah and the Shemoneh Esreh is to show that in Ma'ariv, Semichas Ge'ulah l'Tefilah is not necessary because the Halachah is that "Tefilas Arvis Reshus" -- the Ma'ariv prayer is optional (see 4b and 27b). (This opinion has some practical relevance as well: see Shulchan Aruch OC 236:2 who writes that one may announce on Rosh Chodesh, between Kadish and Shemoneh Esreh, a reminder to say "Ya'aleh v'Yavo." The Mishnah Berurah (in Sha'ar ha'Tziyun 236:4) cites sources that say that one may do this only at night, but not during Shacharis, because Semichas Ge'ulah l'Tefilah is not so necessary in Ma'ariv because Ma'ariv is "Reshus," optional.)

3. Another reason for why Rashi does not mention Semichas Ge'ulah l'Tefilah may be that this requirement applies only when one says Shema in order to be Yotzei the obligation of Shema. In contrast, if one says Shema at a time when he cannot fulfill the Mitzvah of Shema, he also does not fulfill, with this recitation of Shema, Semichas Ge'ulah l'Tefilah. The proof for this assertion is from Rabeinu Tam, cited by Tosfos (2a DH me'Eimasai). Rabeinu Tam asks on Rashi that if we are not Yotzei with the Shema we say at night before the Tefilah (when we pray before nightfall), then this must mean that we are following the opinion of Rebbi Yehoshua ben Levi (cited above in #1). We see from Rabeinu Tam's challenge that even though Rashi agrees that when we say Shema, Ge'ulah, and Tefilah before the time of the Mitzvah of Keri'as Shema, we do so specifically in that order. Accordingly, we in fact are saying Ge'ulah directly before Tefilah. Nevertheless, this is not considered as Semichas Ge'ulah l'Tefilah because one is not Yotzei with this Shema. Therefore, it follows that Rashi could not have said that the reason we read Shema when praying early is in order to be Somech Ge'ulah l'Tefilah, because we are not Yotzei with that Shema and consequently Semichas Ge'ulah l'Tefilah does not apply. (See footnote #26 to Chidushei ha'Rashba, beginning of Maseches Berachos.)

Kol Tuv,

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