Why? St. Lazarus died again, and will rise again.
Well, let's imagine the situation.
Christ dies, and the same time an unknown number of people all around Jerusalem suddenly resurrect and get out of their tombs. People in Jerusalem witness their presence.
Now, that's imagine they were truly resurrected like Lazarus, i.e. only to remain on Earth and die again. If these people were dead and resurrected, their presence in Jerusalem would have been a public miracle including too many people to be denied! Everyone would have attested that these crowd of people was dead and was now living again... that would have been a public manifestation of Christ power who would have embarassed even the authorities... they would have publicly witnessed they were resurrected by God because of the Messiah!
Now, on the contrary, let's take another solution. Those who have been resurrected walk in the city of Jerusalem; they then ascend to heaven and disappear. I can imagine the authorities just talking of "collective imagination" or something like that. Moreover, not everybody would have recognized a dead person revived (I suppose I would recognize my relatives or friends, but if a resurrected person had a fully-restored body he would have been as fit as a fiddle and an unknown person would have passed in vain under my eyes).
I repeat it's only a supposition, and my very-very-very personal understanding of the passage, strictly based on reasoning; and as far as I know this has already been affirmed by other exegetes.
Anyway, I'm not trying to prove anything, dear PeterTheAleut. I only think it's a reasonable possibility. And I told this only to underline how the Dormition (and "Assumption") of the Holy Theotokos has to been understood just as an award for her specific role in Salvation, and also as a promise and figure of the future resurrection of the Christian saints; and not as the RCs state, saying that it is only because of her Immaculate Conception that she was elevated to heaven "at the end of her earthly life".
In Christ, Alex