There are reasons for the absence of textual material, primarily because Mithraism was a secret cult, and outsiders were shunned. But there are other evidences to indicate the pre-eminence of Mithraism. You will need to spend some time here to see why:
I own Ulansey's book. Guess what he says about the dating of Mithraism in the Roman empire? "Mithraism began to spread throughout the Roman Empire in the first century C.E." p. 4.
Now where does he get that date? He guessed. Because we don't have evidence that Mithraism was really around in the 1st century. Our evidence begins at best during the late first, and perhaps early 2nd century.
But even if we posit a mid-1st century date, that's only a little earlier than Paul's death and the composition of Mark.
But as far as any writings are concerned, we do have this as proof of Mithraism's existence in the 1st century BCE:
We don't. Here's the information on the scrap from the the official website of one of the largest collections of manuscripts in the world:
MS 247:
"Commentary: Apart from this MS, no documents or scriptures seem to be extant on Mithra. Scholars have been able to analyze the cult based on fragmentary references, short stone inscriptions, bas-reliefs and sculptures. Mithra was an ancient Indo-Aryan god of the Persians and Indians, identified with the sun, cattle, agriculture, war, truth and immortality. Introduced into the Roman Empire in the 1st c. BC, Mithraism became the most popular and widespread of the foreign religions adopted by the Romans. It lasted until Christianity was adopted by Constantine the Great in 311."
Now, this commentary isn't exactly accurate, in that it reflects the view of the competition between Christianity and Mithras which was the consensus until rather recently. For example, we find in the published lectures (
The Rise and Fall of the Afterlife: The 1995 Read-Tuckwell Lectures at the University of Bristol published by Routledge in 2002) of the noted specialist in Greco-Roman and early Christian religion, Dr. Jan Bremmer, the following:
"It seems, then, that early Christianity had inspired early Mithraism. In light of the most recent insights into the origin of Mithraism this conclusion is less suprising than it might seem at first." (p. 54).
As the death grip of Cumont's groundbreaking but thoroughly dated work was gradually released, along with the increasing realization that Christianity had influenced Greco-Roman religious practice as much as it had been influenced by it, studies of Mithraism in the Roman empire began to note that the figures about the cults membership almost entirely a matter of educated guesses. Additionally, the idea that perceived similarities were the result of Christianity borrowing from Mithraism became increasingly hard to defend. Beck's contribution to the edited volume
Religious Rivalries in the Early Roman Empire and the Rise of Christianity (vol. 18 of the edited monograph series
Studies in Christianity and Judaism; 2006) is informative here: "There is, incidentally, no evidence for the existence of typical Roman Mithraism prior to the very late first century CE. Most accounts of Mithraism place its genesis in the mid-first century CE. My late foundation scenario avoids the awkward evidential silence over the interval." (p. 182).
As the date of the origins of Roman/Hellenistic Mithraism gets pushed back further and further, and the lack of any evidence that it represented a significant "challenge" to Christianity becomes more recognized, the idea that it was Mithraism influencing Christianity, rather than the reverse, becomes harder and harder to defend. The mere fact than the Persian Mithra, who bears
no real resemblance either to the risen Christ or the Roman/Hellenistic Mithras, pre-dates Christianity by quite a bit says nothing about the relationship between the savior deity of the Roman empire.
(According to the author of this blog, we have over 600 papyri as well.)
You didn't read carefully enough. That scrap comes from a collection of 677 papyri which were formerly owned by one individual and have since been distributed to different collections. They tell us nothing about the cult of Mithras.