Berachos 3a

Rabbi Eliezer says that the night is divided into three mishmoros. The gemara says that each mishmor has a sign that signifies when that mishmor is. The signs are: For the first mishmor - the donkies bray; the second mishmor - the dogs bark; the third mishmor - children nurse from their mother, and women speak to their husbands. The gemara asks, "Which part of the mismor is Rabbi Eliezer giving a sign for? If the sign is the beginning of each mishmor (i. e. that the donkies bray at the beginning of the first mismor, the dogs bark at the beginning of the second mishmor, etc.) then why does the first mishmor need a sign? It's at the beginning of the night! (In other words, look out the window and you can tell that it is night time because it is dark!)And if he's talking about the end of each mishmor then we don't need a sign for the end of the third mishmor, because that's the day!" The gemara answers (in its second answer), "Rabbi Eliezer is giving a sign for the end of each mishmor. If you'll ask me, 'Why does the last mishmor need a sign?' (It's the morning!) The last mishmor needs a sign for someone who is sleeping in a dark room, and he doesn't know when the time for reading the Shema is, once he hears wives speaking with their husbands, and mothers nursing their children, he can say the Shema."
One could ask on this gemara, why didn't the gemara give the following answer: It could have said: Really Rabbi Eliezer is talking about the beginning of each mishmor. And the reason why he needs to give a sign for the beginning of the first mishmor, is, because sometimes it becomes very cloudy and people don't know when to say the Shema; but, when they hear the donkies bray they could say Shema? [This is not a far-fetched mistake - the gemara later on (on 27b) brings a story in which a community davened Ma'ariv early because it was so cloudy they thought it was already night.]
I think one may base the answer on a different halacha of Rabbi Eliezer lator on on 9b. "Rabbi Eliezer says: One may read the Shema from when he can tell the difference between Techeiles and Karsi, until sunrise" This is a very short amount of time. Therefore, it makes a lot of sense that Rabbi Eliezer would give a sign to someone who is in a dark room, because without that sign it is very likely that he'll miss the proper time to say the Shema. However, at night, according to Rabbi Eliezer one has a third of the night to say the Shema in. Therefore even if one is not exactly sure if it is yet time to say the Shema at night, all he has to do is to wait a little and then it will surely be the correct time. While if he waits (just in case) in the morning, it is quite likely that he completely will miss the proper time to say the Shema. Therefore, the gemara couldn't say the answer we suggested, because Rabbi Eliezer would not have said it.