Theories about why it did not survive long enough to be known by the early fathers could only be more speculative than threories about its existance, I suppose. It seems reasonable to think that if such a thing existed, and was recognized as accurate sayings of Jesus, it would have been treasured at least as much as Paul's letters.
Absolutely. If it existed at the time Luke wrote his Gospel in the mid 60's AD (or much later according to some scholars) it is hard to believe all memory of it would have disappeared in such a tight knit community just 50-70 years later when Justin Martyr, Clement, Ignatius, Polycarp and possibly Iraneus were writing their works. Very hard to believe.