I'm a bit of a nagger, but I think the question is legit:
I start with an assumption that the formulation of the Mishnah was purposeful and not circumstantial, that Rebbe did put much thought in right wording.
We know that A Succah must be taller than a meter and up to 10 meters high (approx). This fact could be formulated in two ways, a positive - whats's required or a negative - what's forbidden.
The starting Mishnah in Succah is a par-excellence example of a negative formulation:
"סֻכָּה שֶׁהִיא גְבוֹהָה לְמַעְלָה מֵעֶשְׂרִים אַמָּה, פְּסוּלָה. רַבִּי יְהוּדָה מַכְשִׁיר. וְשֶׁאֵינָהּ גְּבוֹהָה עֲשָׂרָה טְפָחִים, וְשֶׁאֵין לָהּ שְׁלֹשָׁה דְּפָנוֹת, וְשֶׁחַמָּתָהּ מְרֻבָּה מִצִּלָּתָהּ, פְּסוּלָה."
A sukkah taller than twenty cubits is invalid. Rabbi Yehudah validates it. And one which is not ten hand-breadths tall, or does not contain three walls, or whose [area of] sun is greater than its shade is invalid.
Rebbe could easily formulate it in a positive way [my proposition]:
איזוהי סוכה כשרה? כל שהיא גבוהה עשרה טפחים ולא יותר על 20 אמה. שיש לה ג' דפנות וצילתה מרובה בחמתה.
What Succah would be Kosher? One that's higher than a meter and up to 10 meters, has 3 walls and has more shadow than straight sunlight.
Please note, that my version is not lengthier and covers exactly the same amount of information.
This negative approach is very common with the Mishnah, for example, "לוּלָב הַגָּזוּל וְהַיָּבֵשׁ, פָּסוּל. שֶׁל אֲשֵׁרָה וְשֶׁל עִיר הַנִּדַּחַת, פָּסוּל. נִקְטַם רֹאשׁוֹ, נִפְרְצוּ עָלָיו, פָּסוּל.". Sometimes the Mishnah does speak in a positive way (RA"S 3,2): "כָּל הַשּׁוֹפָרוֹת כְּשֵׁרִין חוּץ מִשֶּׁל פָּרָה, מִפְּנֵי שֶׁהוּא קֶרֶן."
Personally, as a father and a Mechanech, I'm a big fan of the Positive Psychology approach to education, namely instead of repeatedly reciting the forbidden activities, I prefer to strengthen the desired behavior.
Did someone research on that specific point - how does the Mishnah benefit from such formulations?