It is a known rule that the halacha is like Beis Hillel unless explicitly stated.
The Gemara (Eiruvin 13b) says Beis Hillel and Beis Shammai argued for three years until a bas kol called out and said the halacha was like Beis Hillel. The reason is because Beis Hillel were humble and modest (נוחין ואלובין) and would present the opinion of Beis Shammai before their own.
In Pesachim 114a, it is specifically written that the halacha is like Beis Hillel (והילכתא כדברי ב''ה). The Gemara asks why it was necessary to state this. Isn't it obvious (פשיטא)? The Gemara says you may either say two things:
That this mishnah was written before the bas kol spoke, and therefore found it necessary to state that the halacha was like Beis Hillel.
That this mishnah was written after the bas kol spoke, but we follow like Rabbi Yehoshua who said we never follow a bas kol when deciding halacha, because "the Torah is not in Heaven" (לא בשמים היא). (See Bava Metzia 59 for the whole story there.)
If we don't pasken from a bas kol like Rabbi Yehoshua said, then why is the halacha like Beis Hillel? Can it really be because Beis Hillel had good middos and were humble and modest? And if we do follow the bas kol, why do we still follow Beis Shammai in a number of dinim?