Its' devoid of hell and eternal life because they are a false premise requiring absolute faith. The Gospel of Thomas is about something to be known. That's independent and personally empowering in my view.
It's devoid of heaven and hell because the person compiling the list did not believe in it.
Something which stands out on the front of Thomas, is the assertion that the sayings are "the secret sayings", which implies that the author acknowledges and rebels against the non-secret sayings, which are plentiful in Matthew and Luke.
The dating of Thomas (70-150) suggests that the compiler could be totally aware of other established Christian sayings, which is what he implies at the start. If this is the case, then it's not a set of innocent original quotes.
The analysis of which came first, typically via logia order, and primitiveness, seems to ignore the premise of the gospel.
Your assuming that the Bible came first, when there are a number of scholars who have subscribed to the theory of a Q Document - A source for the synoptic gospels, to explain variations.
This quote is from the Wikipedia article on the Q_Source.
"... This ancient text supposedly contained the logia or quotations from Jesus"http://en.wikipedia.org/wiki/Q_source
(In reviewing this article please note: The Gospel of Thomas was not discovered until 1945
, sometime after this theory was put forward)
What if we take the opposite view, that GOT came first. In this scenario we have a document containing the quotations from Jesus (GOT) and Q theory may be validated.
What then of the missing mystical elements?
Well perhaps they are the man made additions. Why? Perhaps the Romans, still in power long after Jesus was killed, where responsible for the inclusion of mysticism.
The Roman emperors before Constantine were well aware of the benefits from associating the throne to pagan gods. This gave them a god like status increasing their hold over the masses. Do we still think Zeus or Thor et al are valid today?
3 Jesus said, "If your leaders say to you, 'Look, the (Father's) kingdom is in the sky,' then the birds of the sky will precede you. If they say to you, 'It is in the sea,' then the fish will precede you. Rather, the kingdom is within you and it is outside you.
When you know yourselves, then you will be known, and you will understand that you are children of the living Father. But if you do not know yourselves, then you live in poverty, and you are the poverty."
Besides when you come to understand the Gospel of Thomas, Heaven and Hell concepts fail to have any value save for the immature.
Jesus went to the cross probably because he challenged the status quo of the powerful, both the Religious (Pharisees) and the Political (Romans). We see this sort of behavior occurs in history.
It's then not to much of a stretch to consider that his word would have been taboo for some time after his death. The same leaders that sort his demise were still in power after Jesus passed.
There could have been a band of faithful who kept the sayings in secret. This sort of secrecy has happened before through history and particularly around spirituality and leadership.
Heck, even Constantine might have been the driving force behind the shaping of the contemporary bible. Shaping it to suit his political ambitions in the guise of enlightenment.
Just an alternate scenario, certainly no less strange than believing in fictitious tales such as heaven and hell being real.