The mishnayos throughout the perek said that for kodshei kodshim and some types of kodshim kalim (Todah and Ayil Nazir) that are eaten for a day and a night, the Rabbonon made a harchaka that one must stop eating by chatzos as a preventative measure to avoid eating after dawn (as rashi quotes 53a from the first mishna in brachos). However, by korban shlomim and other korbanos that can be eaten for 2 days (and the night in between) we don't find the mishna mentioning any harchaka implying that it can be eaten until nightfall.
Tosafos (57b at the end of the perek) writes that when the end time is the beginning of the night it isn't necessary to make a harchaka because the onset of night is obvious and clearly discernible. But when the end time is the onset of day, which is alos hashachar - dawn, it isn't quite as obvious for the average person to discern and therefore necessary to make a harchaka d'rabonon that it couldn't be eaten after chatzos.
Tosafos considers the onset of night to be easily discernible. If it were dependent on sunset, then this would be obvious. But even Tosafos 56a who holds that the blood of the korban becomes passul at sunset, writes explicitly that the end time for eating korban shlomim is tzeis hakochavim - when the stars come out. Therefore, Tosafos considers it easier to recognize 3 average stars coming out and unnecessary to make a harchaka, than it is to recognize dawn. Apparently, the light of dawn is so slight that the average person cannot recognize it. This would be consistent with Rashi in Megilla who writes that the Rabbonon said that every daytime mitzvah should be performed after sunrise, not after dawn, since it is easy to confuse post-dawn with pre-dawn.