Beyond fifty cubits, it belongs to its finder - חוץ לחמשים אמה הרי הוא של מוצאו
The finder of the ניפול may keep it since there is no מוחזק therefore הולכין אחר הרוב (even by ממונות). However, when there is a סימן there is a תקנת חכמים to be מכריז even when there is רוב that would permit you to keep it.
Majority and closest; follow the majority - רוב וקרוב הלך אחר הרוב
The novelty of s'ר"ח ruling (over the משנה of ט' חניות) is (a) that one רוב is sufficient, (b) there is no קרוב by ט' חניות, and (c) the ruling of הלוך אחר הרוב is even if it is a קורבא דמוכח (as in the case of the wine barrel found in a פרדס דערלה, or the case of the black and white birds).
And if there is no other, it certainly fell from that one - ואי דליכא אחריני ודאי מההוא נפל
We could possibly understand the משנה (that it belongs to the finder) if there was another larger שובך nearby which was הפקר. However, (even) if all the שוכבים are equal it should belong to the most קרוב.
We assume that an item may be from travelers, when we know for sure that the item was lost. However, where there is no such assumption, we negate the possibility that travelers lose their belongings (doves).
He found it between two dovecotes - מצאו בין שני שובכין
The real intention of the מקשן was to (ultimately) ask וליזל בתר רובא דעלמא (even against a קורבא דמוכח), so therefore it did not ask on ט' חנויות (why we follow the רוב there) since there is no קורבא דמוכח there.
And let us follow the majority of the world - וליזיל בתר רובא דעלמא
The גמרא could have answered that there was no other שובכין within thirty ריס, but that would make the סיפא different from the רישא. However, it could have established the רישא where it was found within 'נ and it was מדדה.