Monica's answer is one I would endorse, and this is along the lines of the position of the Chazon Ish and something I heard in Rav Schachter's shiur, that it is based on reality at the time of the Mishnah.
To quote myself:
The position of the Chazon Ish is that reality at the time of the Talmud defines halachic practice. Thus [quoting Rabbi Gil Student]:
Should the halachot of treifot then change to fit the current reality? The Chazon Ish explains, based on the Gemara in Avodah Zarah 9a, that the world was divided into three periods - two thousand years of tohu, two thousand years of Torah, and two thousand years of Mashiach. The full explanation of this passage is fascinating but will take us well off topic. However, the Chazon Ish explained that the halachot of treifot were based on nature as it was during the time of Torah, the two thousand years between Avraham and approximately the close of the Mishna. Since at that time the treifot as described in the Talmud caused an animal to die within a year, we are still forbidden to eat such animals even if they can now survive for over a year. [end quote]
So too (Rav Schachter mentioned in shiur), it might be that we must be koveah the sof zman krias Shema based on when people rose at the time of the gemara.
To explain this idea further, it is not that the Torah wanted to say 6:30 AM or some other specific time celestial time, and used uvkumecha as a convenient way of designating that. Rather, it is fluid, such that any time people are getting up is considered uvkumecha. And, since at the time of the Mishna or Gemara, kings were getting up at that point, that was a fulfillment of the Biblical time. Had there been college students in those days, even until noon would be uvkumecha. But we can add to that the theory that halacha was still fixed at the time defined by the gemara (rather than reevaluating for our days) based on this idea of the Chazon Ish.